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Nsdraftroom > Concepts of Engineering > Ultimax 100 “constant recoil” in AR

Title: Ultimax 100 “constant recoil” in AR
Description: Is it a good indea or not?

Trinary - March 9, 2011 01:37 PM (GMT)
My question is basically what it says in the description, is the patented “constant recoil” action of the Ultimax 100 a good idea in an assault rifle?

I was thinking about using the action in order to increase the controllability of a service rifle during auto fire. The main reason behind this idea is this video.

It just seems so controllable for such a light weapon :)

For those of you who don't know how it works it is basically a simple effort to avoid the heavy bolt group slamming against the backstop in receiver. This is done by elongating the stamped-steel receiver (and available bolt travel path) and by carefully calculating the strength of return spring.

I should probably ad that I intend to have the weapon fire from a closed bolt at least on semi, I am toying around with the idea of keeping in open bolt in full auto (the problem with accidental discharges will be handled trough training, part of the reasoning behind this is actually partially to make the soldiers more familiar with open bolt weapons to make them more flexible without the added hazard of accidental discharge)

So is this a good idea?
How do you think it will affect single shot accuracy?

Two more specific points of interest for me:

I also like bullpup weapons for the long barrel combined with compactness (I would rather not have a discussion of pros vs cons of bullpup except if it has to do with the choice of action) but I am concerned about the travel length of the bolt group. Looking at the Ultimax this does not look like a problem as long as the magazine is quite close to the pistol grip (as it is in many bullpup ARs) I am however concerned how well it will fit with the next point.

I would like to have the AR chambered in 6,5 grendel. And I am concerned how well this will work with the design. In regards to controllability and in regards to actual placement of the action in a bullpup rifle. (Will a stronger spring be enough to keep the increased energy in control or will it have to be bigger thereby making a bullpup unsuitable for this weapon?

Allanea - March 9, 2011 03:48 PM (GMT)
How is it meaningfully much more controllable than this?

Trinary - March 9, 2011 04:46 PM (GMT)
Tbh I think it does look more controlable...

And acording to sevral articles on the Defense Review it is claimed the Ultimax offers an 8:1 hit-ratio advantage over the M16 rifle when both are fired offhand on full-auto side by side against multiple targets and that the Ultimax will will outhit the belt-fed FN M249 SAW and MK46 MOD 0/1 at 3-times the range, on full-auto despite being considerably lighter.

*edit* Link to one of the articles

Trinary - March 10, 2011 03:12 PM (GMT)
Should I interpret the lack of complaints as a okay sign to start designing a bullpup rifle family (AR, IAR and perhaps a DMR) around the Ultimax action?

Satirius - March 10, 2011 04:02 PM (GMT)
The ultimax receiver looks lol long tho

Falls - March 10, 2011 04:38 PM (GMT)
QUOTE (Trinary @ Mar 10 2011, 03:12 PM)
Should I interpret the lack of complaints as a okay sign to start designing a bullpup rifle family (AR, IAR and perhaps a DMR) around the Ultimax action?

Interpret it as a lack of interest in watching videos and reading the articles linked.

Cite the portions of articles that hold relevance...don't expect people to hunt for it.

Also honestly the Ultimax video you linked...most of it is him shooting at what is effectively a building sized target at very close range and he still has a group that is it doesnt show anything very impressive.

Amastol - March 10, 2011 05:02 PM (GMT)
Constant recoils secret is pretty simple IIRC, prevent at all possible any contact with the rear receiver (there by denying the impact recoil usually cause by such action) by means of a long receiver and bolt travel. this means a decent ROF that's cyclically controllable but it also means a loooong receiver and a relatively large amount of bolt travel. This is fine in a LMG (where the ultimax does just fine from what I've read) but it doesn't make it the most accurate weapon out there.

Remember though that an Assault Rifle is by definition and practice a compromise. It will not be as good as an LMG or GPMG in full auto because its usually shorter and or thinner barrel and usually limited magazine capacity, it is also supposed to be lighter and more mobile than an LMG or other support weapon. Notably its accuracy should be greater than that of an LMG due to its tighter tolerances (since its shouldn't be firing off full auto all the time, for the design difference mentioned) in order to provide better point to point engagement instead of area engagement.

So yes you can make the Ultimax style reciever into a AR, just give it a 30 round mag and a lighter barrel and call it done, but understand that at ~5kg (4.9 empty, even the FAL is only 4.45 kg empty) you are going to have to sacrifice somewhere if for no other reason than soldier comfort. Notably you'll be giving up the equivalent of four magazines worth of 5.56 rounds in weight compared to an M16A4. Im not saying that your weapon will be 5kg, simply that the ultimax's only really heavy part is its barrel, and you will only be able to reduce overall weight by so much.

no endorse - March 10, 2011 05:28 PM (GMT)
So we're clear, a spring does not provide a constant recoil force. "Constant Recoil" just means that the recoil isn't a near-instant impulse.

Also, bullpup does not into really long actions.

Consider this a very strong "no" vote, with a side of "no."

Got a fundamental question for you. The FN-FAL, the M14, the 416, the AKM, and the G3 are all fairly good guns. On the bullpup side, the Steyr AUG and FAMAS are good guns. What does yours provide that equals or exceeds these? (note: "domestically designed" is a valid answer)

You're going about this sort of the wrong way, it's a case of the tail wagging the dog. You don't design something and then come up with its role, you need to define an operational requirement and then design something to meet that requirement. For example (and apologies to others for constantly mentioning this gun):

My gun is the NESR-21. Previously, I operated M14s and AKMs in my armed forces due to a fight over 7.62mm rounds and the usefulness of intermediate rounds. (etc etc etc also I was once a n00b) Therefore, I had ample stockpiles of various 7.62mm tooling and ammunition, and a lot of institutional momentum behind "full diameter bullets" of .30 caliber.

The operational requirement was to replace both guns with a common family of firearms that would be (in descending order of importance)
1. Extremely common (units of the family should be as similar as possible for cost reasons)
2. Simple and easy for a conscript to learn to use. This plays into 1, because similar rifles mean simplification of training down to one platform. This is important, as NE uses dumb conscripts during war.
3. Simple to produce and maintain. Important, as NE issues these to citizens when they turn 18, and dumb conscripts have to maintain these during war. We'll be making a lot, and they'll be heavily abused.
4. Reliable: it can't be something requiring constant skilled maintenance
5. Fire a .30 caliber bullet of some sort
6. Sufficiently accurate that a DMR variant could be produced.

Then someone noticed that 7.62 NATO and 7.62 WARSAW can both be fitted to the same bolt face sort of. (don't try it, you'll blow your head off) So the decision was made to produce a family of rounds with the same base dimensions, for bolt commonality. A variable gas delay allowed action commonality, by reducing the delay a lighter round could be fired, and by increasing the delay a heavier round could be fired.

Thus, the gun fires a family of .30 caliber (7.62mm) bullets from a common action with a variable gas delay. A single gun is produced, which is then matched with different barrel/chamber assemblies (similarly to a shotgun) and apropriate magazines for holding the different rounds.

This meets:
1. can't get more common than 1 gun. To switch between calibers, you swap the barrel/chamber and slap in a new magazine. Boom.
2. it's basically a giant HK P-7. Only weird part is the variable gas delay.
3. See the two above. Nothing too fancy here.
4. Gas delayed blowback is stupidly reliable
5. Check, including a 7.62*55mm round, a 7.62*40mm round, and a hilariously hot loaded 7.62*25mm round that's sort of like a wimpy .30 carbine
6. DMR barrels can be produced easily and effectively.

The common cartridge base also means that any round that fits within the length and width of the action that fits a 7.62NATO bolt face perfectly can have a barrel/chamber assembly made for it. Then you just slap in appropriate magazines.

The downsides are:
-the gun isn't the most accurate: the barrel/action mating point has to be manufactured to a tolerance far beyond what I do.
-It's really heavy. The PDW version (firing 7.62*25mmNE) has an action that weighs as much as it should for a 7.62*55mm round.
-Cheap ammo can lead to fouling in the delay tube, bad.
-.30 cal is not the best caliber.

Well, lo and behold someone figured out that the weight sucked, and that no one really liked the 7.62*25mm PDW variant. So we went and designed a new age M1 carbine that fired the 7.62*25mmNE round that weighs less and, thanks to a gas piston, is slightly less prone to fouling.

Amastol - March 10, 2011 06:13 PM (GMT)
QUOTE (no endorse @ Mar 10 2011, 01:28 PM)
Also, bullpup does not into really long actions.

Usually Id agree, the only real caveat would be in a belt fed design ala M60, where you need the standoff anyways.

Trinary - March 10, 2011 06:16 PM (GMT)
*Edit* Long post happend while i was typing this I will read thru it. do check my picure of an ultimax and compare the magazine position to a bullpup with the magazine forward (such as the F2000) I think it would fit

Interpret it as a lack of interest in watching videos and reading the articles linked.

Cite the portions of articles that hold relevance...don't expect people to hunt for it.

Also honestly the Ultimax video you linked...most of it is him shooting at what is effectively a building sized target at very close range and he still has a group that is it doesn't show anything very impressive.

I only put the video in to show were i got the idea from

I did include the important part of the article... That the ultimax is really accurate on full auto.

And i am very much under the impression that he is steering the weapon of free will during the video (I have fired thousands of rounds through both SAWs and ARs and that look like one controllable weapon if you ask me). I am quite sure that that is also the reason that the defence review states that the video seems to be backing up the claims that the Ultimax is really accurate and controllable during auto fire.

Thanks amastol for a good post. I know full well how the action works but you are starting to touch the interesting points.

The lighter weight will of course make it less controllable but read in a magazine (cant remember which sry) that even this version (yes that's an ultimax without stock and with a short barrel) was controllable and useful for CQB. And CQB such as house clearing in where the ARs will mainly use auto fire.
user posted image

The IAR is another story however and since it features longer heavy barrel and bipod I think it could function like a bullpup Ultimax.

I have also checked some lengths and it should not be a problem to fit the action in a bullpup based on the FN F2000.

I do however have a new concern.
Single shot acuracy... In a regular action most of the felt recoil comes when the bolt assembly slams into the back of the receiver (When the bullet has left the barrel) I will this rifle will be receiving more of its recoil when the round is still in the barrel, the difference should be negligible since the bullet is in the barrel such a short time. One thing that supports this view is that a bolt action rifle is not considered inaccurate just because you star feeling the recoil straight away. But still, It might make a DMR version less accurate...

Does anyone have any comments about single shot accuracy? I can also consider using the IAR for DMR duty's like the L86 LSW since it already has the barrel and the bipod for the job just using it on single action (closed bolt)

no endorse - March 10, 2011 06:22 PM (GMT)
Controlability under automatic fire has often been discounted in operational use. But that may depend on how you intend the infantry squad to work.

Amastol - March 10, 2011 06:36 PM (GMT)
I don't think I touched on controlability, Ive always heard its very controlable, I did touch on mobility however; You are still looking @ 4+ kg of 'assualt' weapon even IF you keep the same round (most don't), using a heavier recoiling round (comparatively) is going to suitably increase bolt weight, and receiver length to keep the 'constant recoil' aspect alive at equivalently increased weight.

Trinary - March 10, 2011 06:37 PM (GMT)
Good points about designing for a purpose, I am actually doing just that I simply forgot to tell you.

This weapon came partially from an older post about an IAR where i posted


I want a weapon that's in the middle ground between an IAR and a SAW, that is compact enough to be used by mounted troops (helicopter inserted, sometimes via parachute) whose main purpose is to capture installations (so the will be CQB) the troops will then have to defend the installation from counterattack until help arrives so they will also need to be able to "reach out and touch" the enemy. Cost is not much of a issue since its supposed to be used by the glory boys of the Trinary military/"governmental mercenary service"

So the reason for a bullpup with this action is:
Bullpup for short overall length to help CQB while still maintaining the longer barrel for the defencive situation were range is needed

The reason for the action is:
Increased CQB performance due to more controllable auto fire, and common parts and handling with the IAR (that benefits more from the action) thanks Crookfur for pointing me in the Ultimax direction :)

The reason for the 6,5 bullet is:
increased stopping power especially at range but it wont hurt at CQB either (where the increased wall penetration can be put to good effect)

I am making some sacrifices however:
Price will be higher both due to weapon cost and training all the troops to understand the difference between closed and open bolt action (that should be useful for them anyway)
The weapon will also be slightly heavier than the F2000 that it is based on.
Lower magazine capacity due to the 6,5 rounds that will also be slightly counter productive due to increased recoil

PS: Sorry for not posting why I did it in the first place it makes it alot harder to understand.

Trinary - March 10, 2011 06:50 PM (GMT)
I don't think I touched on controlability, Ive always heard its very controlable, I did touch on mobility however; You are still looking @ 4+ kg of 'assault' weapon even IF you keep the same round (most don't), using a heavier recoiling round (comparatively) is going to suitably increase bolt weight, and receiver length to keep the 'constant recoil' aspect alive at equivalently increased weight.

This is a question don't take it as something else. Do you really think that the action change would ad that much to the 3.39 kg F2000 tactical, that I am basing the design of even with the grendel round? I know the IAR will be heavier (that a good thing) but this is simply an action change, sure to an action that is probably somewhat heavier but still. The round change could also ad some weight but the 6,8mm AUG only weights 0.1kg more than the 5,56 version and (that number could how ever include rounding so 0,05-0,14kg

Amastol - March 10, 2011 07:07 PM (GMT)
The AUG is a pure Gas operated rotating bolt design, which allows for a fairly short operation, and very light weight bolt. This design is a gas unlocked (much like the AUG) but is a rate controlled design that uses a heavier spring and longer receiver length to mitigate recoil impulse and ROF. It may not seem like a major change but since both receiver length and spring will have to be tuned and and in proportion to accommodate the new round (so add the 100 gram increase to that weight as well) recoil force will still play a role, and those dimensions even if all polymer with no metal reinforcement are going to add weight to the design.

Even with rounding yes I still think you'll be looking at a comparatively heavier design.

Trinary - March 10, 2011 07:12 PM (GMT)
I totaly agre that it will be hevier that the F2000 tactical it will be based on but will it really jump upp to over 4 kilos?

Trinary - March 10, 2011 07:18 PM (GMT)
I used the 3,9 kg AK5C during my service in the Coastal rangers and i didn't die lugging that thing around (thou it is a pain being accurate if you have kept the weapon ready to fire for very long periods of time without support or a Chance to lower the barrel for some secs)

My point is that even if we do get to 4 kg its not a dealbreaker.

Amastol - March 10, 2011 07:43 PM (GMT)
Well Im not saying 4kg is where you are at, Im saying 4+ kg unloaded and without sights is not exactly ideal. Some design comparative studies for weight (or at least as accurately as I can do sitting behind my computer.

the Stoner 63 weapon system weighs 3.72 kg empty, was designed by Eugene Stoner and features a 508mm barrel with an OAL two mm shorter than the Ultimax, this length difference could be chocked up to LOP (though maybe not if you have those values on hand). It is a likewise selectable gas operated rotating bolt design.

The Ultimax was likewise designed by a former employee of Armalite, weighs 4.9 kg empty with a 508mm barrel for a 1024mm OAL, and it is a a gas operated, rotating bolt, rate controlled (recoil) design.

Both share very similar outwards appearance, operating mechanism, have somewhat parallel design influences, and have the same relative volume of polymer furniture.

4.9 kg - 3.72 kg = 1.18 kg the majority of that weight being the difference in action weight/receiver addition.

Assuming that weight of action difference is accurate (which it isnt really, its only an approximation since we don't have component to component weight comparisons) a 6.8 SPC chambered FS2000 with constant recoil action should weigh in at about 4.88 kg or so, Say about just over 4.5-4.6 kg since I'm not sure about barrel thickness difference between the designs.

Edit: or just about 6.4-6.5 kg loaded.

edit2 : Also that's the weight of an FS2000 with a 400mm barrel, not 508mm

Trinary - March 10, 2011 07:53 PM (GMT)
I think you have just made a good calculation of what my IAR would weight. Because that is what the Ultimax is (actually a lightweight SAW but wtf) as you yourself said
the ultimax's only really heavy part is its barrel,

I wont say that it is only the barrel that would make it heavier, since they were building a SAW they added things like the ability to change barrel a carrying handle and i don't rely think they were hunting weight that much since if they did that to well it would not really be a SAW at because weight is a good thing when trying to control auto fire.

Trinary - March 10, 2011 07:56 PM (GMT)
Just to point out I am thinking about making a weapon in 6,5 grendel. the 6,8 was simply a somewhat similar rechambering that I knew the weight difference of.

Amastol - March 10, 2011 07:58 PM (GMT)
Remember though the Stoner 63 also had quick swappable barrels since it was a modular design concept. The reason I wasn't sure on the difference in barrel weight is I'm not sure if the Stoner had a heavy barrel by default for its SAW/LMG role or if it had both light and heavy barrels.

6.5 kg loaded vs 5.5 kg loaded with the same 100 rounds for the M27 if an IAR.

Edit: 6.5 Grendel is also heavier recoiling, higher pressure and larger in chamber dimensions than the 6.8 just so you know, those will all effect the weight.

Trinary - March 10, 2011 07:59 PM (GMT)
Why did you calculate the mag weight so high? I am going to use only 26 rounds ber mag (to keep the lenght down, those grendel rounds are fat but short)

*edit* You answerd my mag question there

Amastol - March 10, 2011 08:01 PM (GMT)
Ultimax uses a 100 round drum, so it seemed a fair comparison for either role.

Trinary - March 10, 2011 08:12 PM (GMT)
Well Like I said I think that it is good calculation for the IAR version. Well still a SAW actually but it is roughly the same here anyway the IAR would perhaps be 0,2 lighter or something still not that big of a difference.

But how much would the AR version weight? I just have a hard time seeing that the new action would ad more that 0,5 kilos (I don't have any exact figures but when cleaning my AK5 the entire action barely weights that much And i don't think the action change would more than double that weight. The big things with it are longer range off movement and a heavier spring. Sure the longer movement might ad some weight and the spring will also weight more. But damn those springs are usually featherweight and i find it hard that it would be super heavy all of a sudden.

I don't whant to be unresonable I just find it really hard to think that those two changes could ad that much weight...

Amastol - March 10, 2011 08:24 PM (GMT)
I sadly cant break the weight down any further unless you have weight to weight comparisons of both the Ultimax, and a more similar weapon system than the Stoner 63 which seems to be the closest analogue I can find. the weight ISNT just action however, its receiver and action weight difference. Arguably the difference in weight I gave you was entirely for spring and additional receiver length since both use the same system of locking/unlocking such weight should have canceled each other out.

Is it exact? certainly not, it was a quick guesstimation using quickly found internet resources but is it relatively close? probably, its certainly a starting point. Even still, if you assume the weight is only .5kg heavier, that's still not counting the heavier chamber, additional receiver length to handle the higher power cartridge of the 6.5 so you are still looking at a similar weight difference either way.

Edit: Ah I see one thing that's not clicking between us, Im not saying the spring is super heavy and causing all this weight, but it is part of the added weight. the receiver is adding most of that weight and whatever additional bolt weight is in there as well. Since all we have to go on is empty weight and relative construction/dimensions I dont think I can give much more precise than what Ive given.

Edit2: also the 6.5 Grendel is essentially the same length as the 5.56x45mm NATO round (57.5mm vs 57.4mm respectively) the shorter case length is capitalized to fit a longer and higher BC projectile to improve a 5.56mm chambered weapon while still fitting in the same magwell.

Trinary - March 10, 2011 09:06 PM (GMT)
I fully Understand your reasoning and i think it is hard to find a better analog. But I still think that it is only valid for an IAR version of the weapon. Because we must still keep in mind that they were trying to build a SAW with the Ultimax, so i don't think they wanted to make it as light as an AR.

To give you an indication of the weight difference we are talking about here we can look at bullpup that was made in a IAR and a AR mode;

The L86 LSW weights in at 7.3 kg empty while the regular L85 is 4.13 empty

If we want to look at the effect of simply a barrel change we can look at the heavier barrel of the DMR AK5 which weights 0,8 kg more than the regular AK5 barrel and that is for a DMR not a SAW it should be lighter(not the same need to absorb heat)

I totally agree that this would be much easier if we had a weight breakdown of the parts. But we don't and with my reasoning above I think the weight difference is not a result of the action change.

I have never seen an IAR that is less than a kilo heavier then its parent gun even without any action change. And a kilos difference is exactly what you get with you calculation. If we say that some of the weight difference is still the effect of the action change. Only the heavy barrel should weight almost a kilo but lets pretend it somehow doesn't and the actual action change weights a half kilo (the maximum I can consider remotely possible) we still have and 3,3 + 0,5 + 0,1 = 3,9

Therefore an AR based on the F2000 tactical with this action should weight below 4 kg.

And Even if it did go higher its far from a deal breaker a weapon in the 4-4,5 weight span would be more controllable during auto fire which is one of the points of the design

Amastol - March 10, 2011 09:37 PM (GMT)

I can see your reasoning a bit better now on the SAW/IAR construction concept. The hard part is we are actually going in the opposite construction method as most IAR's go, in that mos IAR's are reinforced assault rifles that are function optimized. Doing it the opposite way is a bit harder since really constant actions benefit is in full auto. Id honestly say why do you need such a function an AR? if you've got a dedicated IAR AND plans for a SAW does your AR really benefit much since its obviously not going to be firing in fully automatically that often (compared to the IAR, and SAW)?

If in essence if all you want is gas operated rotating bolt weapon, then all you need to do is up-chamber the FS2000. In a role where its primarily acting in semi-automatic than in its role it does fine, while the IAR version would do automatic fire better thus justifying its cost/weight. Parts commonality could still exist (use the same bolt, hammer group, receiver, etc) but ignore the need for the heavier spring/heavier bolt carrier/Barrel of the IAR.

Assuming a light 400/406mm barrel, light bolt carrier and spring, in 6.5 Grendel yeah I can see this being around 3.8 kg empty with the added receiver length for use in converting to an IAR simply surplus. Throw in the heavier spring, a heavy bolt carrier (same bolt) and a heavier barrel and yeah I can see it pushing around 4.3-4.5 kg empty. A SAW version would probably require more serious modifications tbh but that's probably for another thread.

The LSW is a stamped metal beast what with the integral bipod and barrel support on top of the longer heavier barrel, so I wouldn't really consider it a good example since it really does add significant material to the rest of the weapon compared to just optimization for Automatic fire.

Trinary - March 10, 2011 10:23 PM (GMT)
I did consider doing it with the regular action only up scaled for 6,5 and that is still one of my options, but I wanted to explore this weapon to see if it was viable (it seems it is) I needed someone to bounce ideas with and you have been a great help!

I am still planing the series and i got a much clearer view of the IAR though this conversation and I also feel more emerged in my AR.

I will probably drop the SAW altogether and use more IARs (still got GMPS with the support group)

I think I will post the writeup stat block and drawing of the rifle series within a week. So you can help me improve it further then :)

Thanks for the help!

Falls - March 11, 2011 03:40 AM (GMT)
nvm, its all already been covered.

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