Roller Delayed Blowback - Its time has come again...

[CmdrSlander]

I was designing guns in my head, as I often do when I'm bored, and the self issued design challenge du-jour was to take a military rifle design and operating mechanism that has fallen out of favor and modify it so that is a fully modern, lightweight, multi-caliber weapons system. I started with the M1 Garand action, gave up on that, as quick, tool-less barrel and caliber changes are not really possible with that design. I contemplated the M1 Carbine design and Mini 14 as well and dismissed for similar reasons. The real problem with ultra-quick barrel and caliber changes, was, I determined, the pesky gas system which the vast majority of modern and vintage autoloading rifles employ, which, by virtue of being attached to the barrel with a degree of permanence make it impossible to remove the barrel as a single unit in one smooth unlock and remove motion.

Then I took a look at the Roller Delayed Blowback operating mechanism employed by weapons like the MG42... its perfect for a modern multicaliber rifle. No gas system means the barrel can be swapped at one's leisure with, in theory, no tools save one's hands. Simply drop in new bolt module and slide in and lock a new barrel and your caliber change is complete, all while using the same receivers, handguard, etc. (unlike an AR which has arguably the quickest caliber swap procedure if one has multiple uppers in different calibers.)

With modern Nickel Boron coatings, which allow for persistent, dry lubrication I think a roller delayed blowback modern combat rifle is quite feasible and would be highly reliable. For some reason, modern combat rifle designers have become obsessed with multicaliber rifles equipped with piston driven operating systems when the mechanism best suited for multicaliber rifle is RDB.

What are your thoughts?

 

[ugaarguy]

I think you should read an HK G3 armorer's manual. RDB is a major pain to headspace.

Also, in a shoulder fired weapon you're going to need a gas system to make recoil manageable for semi-auto fire - why do you think shoulder fired RDB HKs are piston operated?

Last, fluted chambers (necessary for most RDB weapons) are a pain to clean, and they eat brass.

If you can overcome those engineering challenges, and come up with a reliable design that won't cost more to manufacture than current competitors do, then RDB would work. I suspect the above reasons are amongst the many that RDB isn't used.

"For some reason, modern combat rifle designers have become obsessed with multicaliber rifles equipped with piston driven operating systems when the mechanism best suited for multicaliber rifle is RDB."

Probably because it would take tremendous R&D cost to make RDB work as well as current cammed rotation, locking bolt, gas operated designs; and would also cost quite a bit more to manufacture. Which would mean RDB isn't the best system.

 

[Edarnold]

I'm afraid you operating under false assumptions. The quick change barrel of the MG42 is in a locked breech short recoil operated weapon: the roller locking happened to be a convenient way to achieve a high cyclic rate in a gun made primarily of stamped steel. Same goes for the roller locked CZ52 pistol, which again is short recoil operated.

A quick change barrel in a delayed blowback is kind of a contradiction to the principal advantages of that action type: a rigidly mounted barrel that improves accuracy, and a simplified weapon that can be easily Manufactured. The G3 rifle and its many offshoots like the MP5 demonstrate the type in military weapons. The HK P7 and P9 pistols are examples of improved accuracy in an auto loading pistol because of the fixed barrel, which eliminates the complicated fitting issues involved in a moving barrel locked to a moving slide.

IMHO

 

[JHenry]

i still dont really understand the need(well military need anyway) for a multi caliber platform. if i recall correctly the most popular configuration coming out is a 308 that changes over to a .338, sure a 338 is far better at long range but they also want the 308 for its increased availability, but if your going to hump another barrel and bolt(or upper in some cases) why not just bring more ammo? It seems slightly pointless me but it is an interesting new category of rifles. the FN Ballista, for example(so heavy it wouldnt even practical as dedicated caliber platform lol) is cool as hell regardless, wish i could get ahold of one of those.

 

[Shadow 7D]

DUDE
you 'REALLY' need to read about RDB systems, they can be very 'picky' about their ammo
and you are going to SHRED brass, as most multi cal shooters I know also reload, I'm going to say no.

 

[stubbicatt]

Wow. Ugaarguy,

why do you think shoulder fired RDB HKs are piston operated?

I have yet to see a piston on a roller locker.

Really? Shadow,

DUDE
you 'REALLY' need to read about RDB systems, they can be very 'picky' about their ammo
and you are going to SHRED brass, as most multi cal shooters I know also reload, I'm going to say no.

Were one to take an issue G3 or bonafide HK91, and fire whatever NATO standard ammo is available through it, it would function just fine. The roller delayed blowback system needn't be hard on brass, it is just that the only really well known versions of this system, HK, are indeed hard on brass. Were a receiver buffer designed into the weapon, one would certainly be able to create a weapon which didn't mangle brass.

A quick change barrel system that involves a barrel extension of sorts which properly headspaces on a roller type bolt could indeed be quickly interchanged, using, perhaps something like the Masada quick change feature, although while "headspace" in a general sense is probably not the word we are looking for, it sortof applies.

Jhenry, I concur:

i still dont really understand the need(well military need anyway) for a multi caliber platform.

 

[PercyShelley]

The real problem with ultra-quick barrel and caliber changes, was, I determined, the pesky gas system which the vast majority of modern and vintage autoloading rifles employ, which, by virtue of being attached to the barrel with a degree of permanence make it impossible to remove the barrel as a single unit in one smooth unlock and remove motion.

Take a look at the ACR, née Masada, or for that matter any GPMG that isn't an MG3. Having a gas system attached to each barrel doesn't seem to be an unconscionable burden, and in fact it's helpful in some ways. Different barrel lengths and different calibers have different dwell times and pressures, so the gas port needs to be drilled to different diameters to keep the energy delivered to the bolt carrier more or less constant. Since each barrel has its own gas system, they can all be kept adjusted and ready to go when dropped in.

Then I took a look at the Roller Delayed Blowback operating mechanism employed by weapons like the MG42... its perfect for a modern multicaliber rifle. No gas system means the barrel can be swapped at one's leisure with, in theory, no tools save one's hands. Simply drop in new bolt module and slide in and lock a new barrel and your caliber change is complete, all while using the same receivers, handguard, etc.

As noted before, the MG42 is recoil-operated, locked breech (well, mostly. It may unlock so quickly that there's still significant residual pressure in the chamber accelerating the bolt. Chinn's The Machine Gun mentions that a number of recoil-operated autocannons have significant residual blowback pressure, and this additional thrust might partially account for the rate of fire of the MG42.) The HK roller-retarded blowbacks are a rather different animal.

The biggest problem with the idea of a lego-HK (it's been tried before) is the steuerstück, or as it is somewhat misleadingly called in English, the locking piece. As noted in Chinn's The Machine Gun, retarded-blowback systems are extremely sensitive to barrel length. To compensate for variations in barrel length an ammo pressure, HK locking pieces have different contours to tune the operating system. A 5.56mm HK33 with a 15 inch barrel does not have the same locking piece as a 5.56mm HK33K with a 13 inch barrel.

I think you should read an HK G3 armorer's manual. RDB is a major pain to headspace.

Not as such, just... really different.

Because the breech mechanism isn't rigid when its in battery, the bolt will close on a headspace gauge regardless of whether it's in spec or not.

Instead, you have to strip the rifle, flip it upside-down and measure the distance between the bolt head and bolt carrier with feeler gauges. HK manufactured sets of plus and minus sized rollers to open up or close down this "bolt gap" to the appropriate distance. Nothing so involved as re-mounting the barrel or re-cutting the chamber... unless the person who made the rifle really screwed up.

Also, as the rifle wears, the trunnion tends to open up and the bolt gap tends to tighten, so it's not unusual for the rollers to have to be replaced with a bigger set to keep the bolt gap correct.

Last, fluted chambers (necessary for most RDB weapons) are a pain to clean, and they eat brass.

I used to believe this until I examined some brass from a CETME. As far as I could tell, the fouling from the fluting was largely cosmetic, and there was no dimensional distortion from the flutes that re-sizing wouldn't correct.

HKs do have unusually high bolt carrier velocities (about 2X that of an AR-15, as far as I can tell from my amateur attempts at high-speed video-taping) and a fixed ejector, so they do smack the brass pretty resoundingly on the rim during ejection. Aside from greater mangling of the rim and extractor groove though, I don't see any reason why roller-blowback guns would make brass un-reloadable. There was a thread on this very forum a while back with many people chiming in that they do reload their HK brass, and it seems to work fine.

 

[PercyShelley]

Wow. Ugaarguy,
Quote:

why do you think shoulder fired RDB HKs are piston operated?

I have yet to see a piston on a roller locker.

There were a few designs that used rigid roller locking and gas piston operation (I can think of three; one Mauser, one SIG and one Rheinmetall), but these are extremely obscure and nobody has ever heard of them.

HK G3s look like they have a gas system because of the way the cocking tube attaches to the barrel, but there's no gas port or piston there; that tube just houses the charging handle hardware and the foremost part of the bolt carrier and return spring.

As this picture of the MSG-90 shows:

http://i5.photobucket.com/albums/y158/harlikwin/msg90-2.jpg

The cocking tube doesn't need to touch the barrel at all.

 

[Ian]

Yeah, Mauser experimented with roller locking and gas operation in 1944 (I'm actually editing together a video on this gun - the Gerat 06 - this weekend). German ordnance rejected it because it wasn't significantly better than the StG44. Mauser's response was to invent roller-delayed blowback specifically to get rid of the need for a gas piston, and that made for a lighter and cheaper gun.

IMO, roller-delayed blowback is a good system for intermediate cartridges, but not so great in full-power shoulder rifles.

 

[ugaarguy]

I have yet to see a piston on a roller locker.

My bad - memory was a little foggy posting in the middle of the night.

I used to believe this until I examined some brass from a CETME. As far as I could tell, the fouling from the fluting was largely cosmetic, and there was no dimensional distortion from the flutes that re-sizing wouldn't correct.

One, I'm talking about fouling in the chamber itself, not on the brass. Two, I also owned a CETME, and the dimensional distortion from the fluted chamber made the once fired brass unusable on my example. Resizing was attempted on my RCBS single stage with RCBS dies, and a friend's Hornady press with Hornady dies. It just goes to show how inconsistent fluted chambers can be.

Because the breech mechanism isn't rigid when its in battery, the bolt will close on a headspace gauge regardless of whether it's in spec or not.

Instead, you have to strip the rifle, flip it upside-down and measure the distance between the bolt head and bolt carrier with feeler gauges.

Which is exactly why it's a poor choice for a field serviceable quick change bbl system.

 

[Z-Michigan]

the pesky gas system which the vast majority of modern and vintage autoloading rifles employ, which, by virtue of being attached to the barrel with a degree of permanence make it impossible to remove the barrel as a single unit in one smooth unlock and remove motion.

Not true of many good and simple gas piston systems. I own a Robinson Arms M96 rifle, which was on the US commercial market from roughly 1997-2005, and to remove the barrel and install another all you need to do is:
-lock the bolt back
-push in a button the underside of the receiver
-pull out the barrel, which has a gas block permanently attached but no other parts of the action
-insert a new barrel into the receiver, let go of the release button and let the bolt forward.

This could be done in under 10 seconds if you had the new barrel handy. It seems to be completely reliable and durable. The only real limitation is that the gas port has to be in the same place on each barrel since the piston length is fixed.

 

[Shadow 7D]

M240B
takes less than 5 seconds to change a barrel (cold - hot you got to handle a lot more carefully)