TFBTV Weekly Episode 2: How Open Bolt Guns Work & Roller Delayed Maintenance

Several people last week asked about how open bolt firearms work, so in this episode we show the difference between two very different Uzi pattern firearms. We also get into how to keep your HK style roller delayed blowback guns in shape and running smoothly.

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The full transcript …

– Hey guys, it’s Alex C with TFBTV.

Welcome to another episode of TFBTV weekly.

Today’s questions were fielded by you guys last week.

If you have questions for next week, make sure to put them in the comments below and we’ll take a few and address them.

But one question we got was about, “when should you replace the rollers in your “HK roller delayed rollback gun or style gun?” That’s gonna be your PTR 91’s, your HK 90 series guns and so on and so forth.

Especially with the new Zenith guns hitting the market and whatnot, there’s a lot of these guns out there so you’ve got three major calibers for these guns.

You’ve got nine millimeter, you’ve got.223/5.56 and then.308/7.62×51.

With the 7.62×51 and the 5.56 guns I like to check my bolt gap about every oh I’d say, 2000 or 3000 rounds.

That doesn’t mean it’s gonna be out of spec but technically in spec is gonna be bolt gap of .004 to.020.

I would suggest anybody that owns a PTR 91 or an HK roller delayed blowback gun, pick up a feeler gauge from from your automotive parts store and always keep it in your range bag just in case.

Always keep it at least in your parts room, gun cleaning room just in case.

So use that periodically, every 2000 or 3000 rounds.

I like to keep my bolt gaps about .012,.013, somewhere in the middle.

There’s a lot of guys, a lot of experts who say “well you should keep it higher,” and “you should keep it lower.” I’ve always had good luck keeping it pretty much in the middle.

If you have a gap closer to.020 it’s gonna affect the way your gun hits you.

I’ve also seen bolt gaps completely disapear and actually blow magazines apart if basically the action opens too fast, vents the gas down, blows the magazine out.

So you don’t want that to happen.

Not that PTR 91 magazines are expensive but still that’s pretty nerve racking that that could happen.

Although it was pretty funny when it happened cause all the guy’s ammunition just gracefully fell out.

Yeah, so that happened but don’t let it happen to you and check your bolt gap.

You can order plus 2, plus 4, plus 6, and I believe there’s larger plus 8 rollers and so on and so forth, from Great Sources HK Parts or Robert RTG has them.

Make sure you do that, it’s also worth noting on nine millimeter style guns, I’ve got an MP5 and an MP5 clone.

I’ve actually never had to replace the rollers and I’ve shot 10,000 plus rounds, nine millimeter just doesn’t hurt the receivers in the trunnion as much as basically your higher pressure rounds that really do take their toll there.

So that’s worth noting, I’ve never had to change my nine millimeter.

It’s a good idea to check and I have checked periodically.

But basically if you don’t know what happens, the trunnion gets a little worn out with use and the rollers expand it, keep the bullet gap in spec, keep your bullet from prematurely opening and yeah, keep your HK gun maintained and happy.

You know, whereas an AR15 actually self head spacing and are prone to cracking bolts with high round counts.

I’ve never actually seen an HK bolt break but I have seen some problems as a result of people not maintaining their rifles.

So always maintain your rifles but another question today we got was “how does an open bolt firearm work?” So I rounded up a couple Uzis for you guys and I’m gonna show you the difference between an open bolt and a closed bolt gun, so you see how that works.

Alright guys, so what we’ve got here are two different Uzis.

Well technically they’re not really Uzis because they’re not Israeli guns and they’re not actually branded with the name Uzi.

Instead they’re humorously called, Group Industry/Vector HR4332’s.

Now H.R. 4332 is actually, this is kind of funny, that’s actually going to be the amendment to the 1986 FOPA that banned, or closed, the machine gun registry.

So I guess Group Industries/Vector named the HR4332 as kind of a slap in the face to that whole bill.

Which is pretty funny if you think about it.

But we’re going to cover what makes these two very different.

One is a submachine gun and one is a semi-automatic carbine.

Obviously for people that don’t know, machine gun, hold the trigger down, it’ll fire until you let off the trigger.

Machine guns are legal in the U.S.

under a specific set of circumstances, but no new machine guns can be manufactured or well registered for civilian sale so we can only trade the ones that are on the market.

I can explain how to purchase a machine gun legally if you’d like me to in another video but that would be another video so let’s go into how an open bolt submachine gun works.

Now I’ve got both of these on the table to show how very different these are.

Now the top one here is gonna be semi-automatic only.

One of the dead giveaways is actually the slot cut into the bolt that the submachine gun does not have.

Now it’s worth noting that some upfitters in the U.S registered some slotted bolts that they converted to drop into semi-automatic Uzis to be full auto.

So some of the full auto Uzis in the U.S. you see are slotted bolt conversions which is kind of cool cause you can take the bolt out, put it in another carbine and then you essentially have an unlimited amount of hose but Uzi receivers are rigid, I know of one that has well over a million rounds on it that are documented and there’s been no problems.

Anything that breaks really you can fix and or repair in no time at all.

So with that, let’s have a look.

This ones gonna be open bolt, that means (click) if you see the bolt open, if you see a window there, the gun’s ready to fire.

Whereas a closed bolt, (click) you cock it and it’s ready to fire too but the bolt it closed.

Now open bolt firearms are legal in the U.S.

but they were banned in 1982 because the ATF deemed that they were too readily convertible to fully automatic, which is partially true.

If you ever look at one of the old open bolt MAC-10 semis it doesn’t take a genius to look at it and say, “hey I bet I can turn that into a machine gun.” But don’t do that, that’ll cost you some time.

So let’s take a look at what makes em tick.

The open bolt gun is actually a lot simpler than the closed bolt gun.

Now, what happens here is when I pull the trigger, (click) the bolt slams forward and would pick up a round off the magazine, push it into the chamber and fire.

The way that works, is you got your bolt here, I’ll hold the trigger.

Basically it just reciprocates the longer you hold the trigger down.

You let off the trigger, it would catch and you pull it, keep cycling.

So let’s go ahead and remove that bolt.

Uzis are brutally simple firearms, you gotta love that aspect of these things.

You got your feed ramp here, and it feeds from the magazine into the feed ramp which goes into the barrel, which gets set off by the bolt.

Now the bolt Let me zoom in real quick.

Now the bolt has a fixed firing pin right on the bolt face.

And that’s because when the bolt sweeps forward it picks up the round feeds it into the chamber and as soon as the bolt strokes all the way forward and sets into the barrel it ignites the primer and sets the round off which kicks the bolt back and if you let off the trigger, holds it to the rear.

Very simple, not all open bolt guns have a fixed firing pin.

Most of your center fire stuff is, sorry, most of your rifle cartridges, like FN 249s and stuff like that are not going to have a fixed firing pin.

So it’s mostly relegated to submachine guns and stuff like that with limited exceptions but that’s how an open bolt Uzi works.

They’re brutally simple.

Just a stamped sheet metal receiver.

Some other cool features about the Uzi that I’m gonna go ahead and note, is that they have a quick change barrel.

Something that seems to be lost on modern submachine guns like the MP5 and KRISS Vector.

I really wish the MP5, could do that instead of you having to send it off to have a pin pressed out, have a new barrel drilled, a new barrel pushed in but you know, I digress.

There’s probably a dirty reason that manufacturers don’t do that anymore, I would suspect and that’s because they would like you to send your firearm in for repairs so they get the money but I don’t know.

There might be design reasons and whatnot for that.

I’m sure there are but it’s kinda funny, the Uzi, knocked that one right out of the park in my opinion.

And that’s also a feature carried over into the semi-auto Uzi.

(click) Now, the semi-auto Uzi closed bolt gun, you’ll notice has a lot more stuff goin on the inside there, it’s got a whole separate striker assembly and spring and whatnot and then when I pull the trigger, (click) that actually is what goes forward and sets the round off.

Now if I remove everything, you can see that the firing pin there is not part of the bolt face so the bolt still does the deal where it sweeps forward onto the magazine, strips a round off, goes into the chamber.

But until you pull the trigger, which lowers the sear off of this surface here and sends the whole firing pin forward it won’t go off.

There are also some closed bolt fully automatic conversions I’ve seen.

I don’t have one here, I’d show you how that works as well.

But it is a lot more complex than the open bolt Uzi.

So basically that’s the difference between an open bolt and a closed bolt Uzi.

Other than that they’re remarkablely similar guns.

The semi-automatic Uzi receiver, they’re kind of famous for having this restrictor ring here so you can’t put an SMG barrel in there.

Most people will either mill that off or cut a barrel down like I did.

This is a registered SBR by the way.

You also have a buffer in the back that you can put in full auto Uzi to increase the cyclic rate.

And then they have this blocking bar here that keeps you from putting an SMG bolt in there.

So for example, if I try to put my Uzi SMG bolt it won’t go past that blocking bar.

So that’s why like I was mentioning earlier upfitters slotted bolts and did the full auto conversion is basically they just slotted this to surpass that bloacking bar.

But other than that, that’s the main differences there.

They look very similar but they function very differently actually so that’s a rundown of how this system works.

Most open bolt submachine guns function the same way but lets go back to the table.

Alright guys, I hope that was nice, educational, and informational.

It was kind of redundant for me to do that, I’ve actually written an article on it in the past but people asked so we delivered and I hope that you now are kind of familiar with how open bolt guns work.

What I would suggest is if you go to gun shows anytime you see a MAC-10 or anything like that, always check to see if they’re open bolt guns because if they’re pre-ban, pre-82 open bolt guns, they’re actually worth a lot of money and you never know, the person who has it might not know what it’s worth and you could luck into a pretty valuable gun.

Which is cool, it functions kind of interestingly.

So something to look out for guys.

But other than that most of your open bolt guns on the market are gonna be registered machine guns.

Just the way it is.

So yeah, that’s pretty much all there is to that.

Anyways guys, like I said in the beginning, ask your questions, get em in next week and we will address them and fill a new segment for this.

So we appreciate you guys watching TFBTV weekly.

Last week went great so we’re gonna continue the series.

Anyways we really appreciate you watching, we also would like to thank Ventura Munitions and Grizzly Target for helping support the program, otherwise there’s no way we could do all this.

But anyways I’m Alex C with TFBTV.

Thanks for watching again guys we really appreciate it.

If you hit that subscription button it’d help us out too so we hope to see you next week.



Steve Johnson

Founder and Dictator-In-Chief of TFB. A passionate gun owner, a shooting enthusiast and totally tacti-uncool. Favorite first date location: any gun range. Steve can be contacted here.


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  • Badwolf

    i want to know how an hk g11 works

    • Pretty simple really: The original guns had a base-plate of prefabulated amulite, surmounted by a malleable logarithmic casing in such a way that the two spurving bearings were in a direct line with the pentametric fan. The main winding was of the normal lotus-o-delta type placed in panendermic semi-boloid slots in the stator, every seventh conductor being connected by a nonreversible trem’e pipe to the differential girdlespring on the ‘up’ end of the grammeters.

      • JimBob

        Just say “it’s magic.”

  • Ed

    Do a comparison with a Full auto M-3 SMG and a Semi Auto version.

  • imbecile

    The Benelli inertia action patents have run out. Is there anything interesting happening with it? Are there guns available or in planning that are not shotguns for example? Would it even be feasible for rifles or even handguns?

    • iksnilol

      Benelli MR1 I believe uses the inertia system. It is a 5.56/.223 autoloader using AR mags.

  • JimBob

    A two tone Rolex. Really?

    • My grandfather left it to me when he passed away. I wear it every day in his memory.

      • JimBob

        That’s pretty sweet. A bold choice for hand model work though.