Inertia unlock for handgun?

[Gabe]

I'm trying to picture a handgun that works along Benelli's inertia unlock principle. The advantage would be fixed barrel and a very strong locking action. Let me know what you think.


From the outside the inertia operated handgun would look like a conventional automatic. There are three main differences:

1. There are two seperate springs, a conventional recoil spring, and a larger inertia spring which wraps around the former.

2. It use a two lug rotating bolt which locks into the fixed barrel. When the gun fires the bolt remain locked to the barrel until the blowback subsides.

3. The slide is free to move forward a fraction of an inch.



When the gun is fired it begins to recoil, but the heavy slide resist this motion by staying put briefly. Therefore like the Benelli action, the slide compresses the inertia spring, which kicks the slide rearward. The bolt would have a curved cam track which the retreating slide catch and rotates.

With the bolt unlocked residual pressure forces the bolt back along with the slide, completing the case extraction sequence. Finally, like a conventional automatic, the recoil spring acts to pull the slide forward, picking up a fresh round, and locking the bolt into the barrel.

 

[Jim Watson]

The Benelli (Fancy that!) B76 operated on an inertia lock but it did not invove a rotating bolthead. It had a toggle internal to the slide. It worked ok but was a commercial flop because 1. It had a steep grip angle a 'la Luger, which felt sexy, but did not feed well with anything but ball ammo. 2. It was a single stack gun when high caps were IN.
Wish I had bought one when the importer was blowing them out cheap.

The Wolf Ultramatic was probably classified as an inertia action, but had a lockup based on expansion of a collet-shaped bolt head, not rotation, either. It was a flop because it did not work. They signed up to sponsor a pro IPSC shooter to demonstrate its speed but they could not make it run well enough to compete, even in a tuned match gun.

I thought the Gabbett-Fairfax Mars was more nearly long recoil operated. The operating stroke was so long that the fresh round was drawn out of the BACK of the magazine and then fed across a long jump to the chamber.

 

[mete]

One of the finest pistols ever made was the Remington 51. This had an inertia or delayed blowback system . Produced in 32 and 380 but a protoype in 45acp was also made. It was a Pederson design.

 

[SelfProclaimedExpert]

Gabe,

I've wondered about the same. The Benelli shotgun system is very elegant and simple.

The curiousity here is why Benelli chose a delayed blowback system (I don't believe it's inertial, Jim) instead for the B76 series. A technical limitation?

The main problems I could see having with such a system in a handgun are:
a) Mass. For the inertial unlocker to work reliably it has to have enough mass to both overcome the spring pressure AND (more importantly) overcome contact friction with the parts it rides in and on. A 9mm bolt head in a medium size pistol will require an unlock mass much smaller than what's found in the shotgun.
b) Muzzle rise. Given the overall mass and length of a shotgun, it tends to recoil more directly aft. A 9mm pistol tends to have alot of muzzle flip, by contrast. That flip may translate into too great a side load for the inertial mass to slide forward properly. Of course, with such a system you could make a very low bore height, mitigating this.

Are you actually building this thing?



Flyer, what principle are you using to lock the Mak? (If you'd care to say.)

 

[Jim Watson]

RE Benelli B76 I think it is peculiar enough that defining the operating system is down to semantics and choice of terms. Unfortunately it was imported kind of under the horizon by Sile while H&K was selling Benelli shotguns, so there was not a whole lot of coverage. Sile called it "locked breech," J.B. Wood called it a "hesitation" action, and Donald Simmons hedged by saying it was "recoil - blowback." Close to a Remington 51/53 but with the extra part to hold the breechblock in position just long enough. Simmons said there was enough residual chamber pressure at the time the toggle let go that it would operate without an extractor, typical of blowback.

I don't take terminology too seriously. I have seen Beretta company literature describing the M-92 as "delayed blowback." The Benelli shotguns have themselves been described as delayed blowback. Inertia locking may just be an advertising phrase.

RE Gabbett Fairfax Mars The only source I have on them is Geoffrey Boothroyd who summarizes the patent description like so (My comments in CAPS.):

"Fig 1 shows the mechanism immediately after the discharge of a cartridge. The barrel and breechblock recoiled for 3" (THREE INCHES!) compressing the recoil spring housed under the barrel and also two light springs which acted as breechblock return springs. As the barrel and breechblock started their return journey under the influence of the three springs, the bell crank was depressed by an actuating rod operated by the returning barrel. The bell crank forced the stud downward, rotating the bolt head anti-clockwise through 45 degrees so that the locking lugs on the bolt head were disengaged from their recesses inside the barrel.
NOTE: THE ROTATING BOLT DID NOT UNLOCK UNTIL THE BARREL & BOLT STARTED BACK FORWARD! DOESN'T SOUND LIKE DELAYED BLOWBACK TO ME.
It can be seen from Fig 2 that as the barrel and breech block recoiled the top cartridge in the magazine was drawn backwards by the carrier, the top of the magazine having a metal guide to prevent the cartridge from being drawn forward. As the carrier rocked upward, the fired case (which had been retained against the face of the breech) was ejected by the carrier and the new cartridge was retained in the position shown in Fig 2 until the trigger was released. The bolt, in turn, was then released to chamber the cartridge as in Fig 3. KIND OF LIKE SOME SAVAGE .22 AUTOS, WHOSE BOLTS DON'T CLOSE UNTIL THE TRIGGER IS RELEASED.


I think the reason you don't see more handguns with rotating bolt lockup is that they have to be so big to hold all the machinery. I know of the G-F Mars, the original Auto-Mag, the Wildey, and the Desert Eagle; all great big guns.

 

[SelfProclaimedExpert]

Jim,

I keep thinking of getting a Benelli pistol just to see exactly how it works. My understanding is that it is an actual blowback delay mechanism.

I don't think the various definitions are just rules of thumb. While they're often misused, they have concrete and useful meanings that should be applicable to any design.

Blowback and delayed blowback are pretty easy to identify with a simple test: If the action can be opened by the casing pushing itself out of the chamber (the very meaning of "blowback"), it has to be one of these.

Every other type of action (recoil, gas, inertial) requires a force external to chamber gas pressures to actuate it. These guns have true locked breech chambers.

You can demonstrate this difference by taking hold of the barrel only and attempting to force the action to open by pushing a dowel down the barrel against the breech face. If I haven't misunderstood the Benelli 76, it will open up under these conditions. A Beretta 92 will not.

The only gun which really seems to be on the fence is the FN FiveSeven, but it still requires the barrel to recoil to unlock from the slide, it just isn't employing a massive slide to time it - it uses barrel drag instead.

 

[BigG]

I think it sounds like a pretty good idea, Gabe! If you can build something like this I think it would be a winner.

The only downside to it from my point of view - I have the 1201FP Beretta shotgun (same action as Benelli) is the enhanced kick afforded by the unique mechanism.

A box or two of shotshells thru the Beretta is about all the recoil these old bones can absorb in a day on the clay courts.

 

[Jim Watson]

SPE,

Look for a 1984 Gun Digest. It has the only serious article on the B-76 that I know of, with drawings of the breechblock and toggle layout. By your definition, it would probably be rated as delayed blowback, although they did not do the ramrod test with support on the barrel.

 

[SelfProclaimedExpert]

Thanks, Jim. Used bookstore time. I do have a disassembly manual featuring the gun, but it doesn't tell a very good story.

 

[Gabe]

Thanks everyone for the positive feedback. The Benelli action is truely beautiful. I came up with a few variations, but this is the simplest I can think of. I'm going to check out the Mars and B76 action.

No I'm not building this thing. I wouldn't know the first thing about building something mechanical, but hopefully one day. So if anyone of you want to give it a try, go on ahead. I'm just doing a little mental excercise trying to envision how each and every action can be used in each and every firearm. I don't have anyone to tell me which are the blind alleys, so I'm working my way down all of them. Without an encyclopedia of gun actions, I'm probably not as systematic as I'd like.

I don't think a rotating bolt neccessarily has to be oversize. Rather since they are usually used for high pressure cartridges they are made oversize to deal with the recoil. The inertia operated pistol should be able to handle the most powerful pistol cartridges, but honestly I can't imagine a huge demand for that kind of gun. Unless the automatic can significantly reduce recoil, people would probably stick to revolvers for the bigbores.

For medium powered cartridges, this design should be more accurate and stay accurate longer with the fixed barrel. But other than that it'll cost more and I don't know if the advantage would justify it. Unless there's some +P+++ people are dying to try.

BigG,

I really haven't thought about it, but does this action make the felt recoil worse than a pump?

 

[BigG]

I never thought pumps kicked that bad. I usually fire fixed breech doubles or O/Us. Just seems like the Benelli action has a little more kick than the others. Subjective, you know. Could be stock fit or somethin.

 

[RON in PA]

SPE: I think you have the dowel test all wrong. Try it in a Walther(P-38 or P-92)lockup or any Browning tilt barrel lockup and the barrel and slide will separate after the two have traveled together a short distance (short recoil mechanism). True blowbacks have a fixed barrel and generally work well only with short, straight walled, low pressure rounds.

 

[SelfProclaimedExpert]

Ron, you're not doing the test right. You have to hold onto the barrel, not the frame. I realize holding a 1911 barrel wouldn't be easy, but it's really a mental test.


Flyer,
I see a couple potential problems, but it certainly is a neat idea.

1. Your mechanism, as described, sounds like it actually causes the slide to pause, rather than just slow it down. If that's the case, once the pressure drops, what's going to get the slide going again? There are no recoiling masses on the move, and the blowback pressure is gone.

2. If the mechanism merely slows the slide, it's doing it through friction by rubbing against the inside of the slide. The longer it rubs, the smoother the contact area and friction (and delay) decreases.

3. If it does work, as a mechanism that relies on both friction and spring pressure (rather than trapped lever arms), there is no guarantee that it will grab, every time. If it doesn't happen to properly engage, you now have a 35,000 psi straight blowback gun. The chamber will open too early and the case will rupture. Brass in your face, mag and extractor blown out, big danger.

 

[RON in PA]

SPE: went back an reread your post and thought about it and you are right. It really is a "thought" problem, just about impossible to do with a real gun.

 

[SelfProclaimedExpert]

Well, all you need is an exposed barrel. So you can do it with a P-38, Luger, a G3 rifle, a Ruger MkII, etc. For those you would get no, no, yes and yes - locked, locked, blowback and blowback.