Not sure if this is the right place for this topic:

I was wondering if anyone knew of a semi auto Ultimax 100 ever being built from a parts kit, or if it’s even possible, i.e., if there is an “approved” design for such a build. The Ultimax 100 fires from an open bolt, which I know makes things a bit… complicated. But I know there are some semi-auto builds made from parts kits of open bolt guns.

Much appreciated.

Never heard of it.

Even the post-sample Ultimax MGs are uncommon in the US, so I doubt you'd find enough parts, unless you bought a whole post sample gun for parts, and I rarely see them offered for sale.

Have not seen (nor heard of) an approved semi-auto design either, but I'm sure that's doable.

It's a cool gun though. I got to shoot a MkIII a few times.

Thanks for responding so quickly. Sick video! Judging by the footage, I take it that what people say is true, there is virtually no recoil?

I’ve never seen one in person but always thought they looked really cool (which is what’s most important, obviously, lol), and found the constant recoil intriguing. Although I suppose that’d be moot on a semi auto build.

I've never seen a kit but I'm sure the tried and true stuff would be OK.

Make sure it is denied so unmodified parts cannot be installed. Generally a striker or hammer fired setup is OK with the denials. Original FA parts must not be able to be installed.

Me either, until recently! I’ve seen two in comparatively quick succession on GB: https://www.gunbroker.com/item/1034418057

Thank you for the info. That’d be a cool project.

I remember as a kid, well in high school and longer ago than like to admit, seeing an ad for parts kits in an issue of Small Arms Review for $1k, haha!

Always wanted one.. Always knew they would be expensive due to its rarity.

I've posted this on the M16 section and this may interest you: https://c3junkie.com/?page_id=1403
As you probably know, Jim Sullivan is the inventor of the Ultimax and also the inventor of the Surefire OBC where he states in the interview on Forgotten Weapons that he was trying to get close to the constant recoil of the Ultimax in the M16 with minimal changes to the gun.
I took it a step further with more changes and achieved a state where the carrier doesn't bottom out like the Ultimax and it is super smooth.
I'm sure not as smooth as an Ultimax since my setup is closed bolt not open bolt.  I only fired an Ultimax once and it was amazing.  Maybe one day I can compare to my setup.



A positive side effect to this build is that since there is such a long delay that there is almost no gas to the face when suppressed as the large volume of the fat gas tube has to fill up before the bolt unlocks giving more time for the gas to go out the muzzle like it should.

Video below is suppressed and you don't see any gas coming out like you would on a normal AR setup:



I also TIG welded my own large volume gas tubes for the Mid length for an even smoother operation here:


I did some Rifle length but I can't the pictures right now of the results.

Any of the "no recoil" benefits would mostly be ruined by converting it to closed bolt semi auto.

It would be just as good to build an AR with amphibian's findings.

Hey sorry for the delayed response. Thanks for sending that along. I might try that in my next build, or mod one of my other builds.

That parts kit sold for $4500 on GB, which seems nuts. “This is why we can’t have nice things, Lana.”

Yeah, the constant recoil thing is great. It works very well. Sullivan definitely knew what he was doing.

Yeah that's certainly a lot.

Have you thought about what you'd do for a receiver, if you were able to score a DEMIL'd kit like that?

I agree that an AR running a constant recoil setup like Amphibian has would be the most cost-effective solution.

I've got an AR HBAR build I've been working on. Snagged one of the old Colt LMG hydraulic buffers (and its green spring) but haven't got it running yet. Amphibian's solution is probably the better option today.

I also bought a SAW. More than 4.5k but at least it comes with an intact receiver!

No constant recoil though. It's so violent, in the early 90s somebody was like, "Hey we should add a hydraulic buffer!"

I don't understand why Surefire discontinued the OBC so quickly.

I read on another thread recently that it was discontinued because it was breaking bolt catches due to impacting them at a higher velocity (because the longer aft travel = more room for acceleration when moving forward).

Are you referring to this thread?: https://www.ar15.com/forums/Armory/I-m-designing-a-5-56-belt-fed-upper/65-551641/
The guy said he didn't know if it was true either.
I still run my 'Ultimax' setup and not worried about catches breaking in my setup.  I don't think it is going to happen considering how soft mine is shooting and also using a hydraulic buffer is going to cushion the little impact there is on the BHO.

Yeah, that's it.
Idk if it's true either. Never even seen one in person.