Differences in blowback and delayed blowback (Question #2)


January 19, 2005, 01:22 PM
Here's a few more questions concerning the blowback action of handguns:

First off, is the pressure against the breech face from the case head of a detonating cartridge generally sufficient enough to cycle the slide? I am under the impression that it is, however I have also been told that it is typically the inertia of the accelerating bullet that initiates slide cycling.

Second, can a blowback pistol possess a heavier spring in lieu of a bulkier, heavier slide?

Also, what exactly makes the difference between straight blowback and a delayed blowback? If a delayed blowback functions exactly as it sound (a slight delay in slide operation), how is it that the cartridge case head would still possess enough rearward thrust to cycle the pistol? Is it necessary for these delayed blowback designs to have a massive slide such as the blowback, or can it have a svelter slide similar to a locked breech?

Finally, would it matter to you terribly if a potent-caliber handgun featured a delayed blowback instead having a locking breech?

For example, say there are two similar pistols of the same weight, accuracy and reliability... identical in every way except one has a locked breech and the other a delayed blowback. Which would you choose and why?

Thanks in advance!

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Jim Watson
January 19, 2005, 04:45 PM
First, if it helps clarify, think of a blowback action as gas operated with the cartridge case serving as the piston. Chamber pressure drives the bolt or slide back. That is adequate, many movie prop guns have been altered to blowback operation and have a barrel restricting bushing to hold enough pressure from a blank to function the slide with no bullet in sight.

Second, spring strength is mostly a fine tuning mechanism, it is the inertia of the slide that resists chamber pressure. Casehead thrust of a .22 long rifle is about 700 peak pounds. You aren't going to stop that with any spring you can cock the gun against. There were some designs that disconnected from a super stiff recoil spring for loading but they were kind of marginal.
You mentioned the Hipoint. Cheap.
On the other hand, an Astra 9mm is about the most powerful blowback pistol in actual miltary issue and nicer made than most other Spanish pistols. It has a fairly heavy slide, a strong recoil spring, and a strong mainspring, cocking of which resists slide movement a good deal. Everything has to be factored in to a real design.

Delayed (or retarded) blowback is done by a variety of methods. The 1905 Mannlicher had a cam in a notch in the bottom of the slide. It took extra force to drive that cam out of the notch, in addition to slide weight, recoil spring, and mainspring. The H&K P7 has a gas cylinder piston under the barrel. As soon as the bullet leaves the cartridge case, the cylinder pressurizes and the piston adds its resistance to the recoil spring and slide weight. The FN Five-seveN is delayed blowback but I have not seen a description of just how it works.

As far as choice, the P7 is a good shooting gun. I sold mine because I could not get used to its squeeze cocking action unless I quit shooting all other action types and I didn't want to do that. The slide function made no difference.

I would not know how to choose between recoil and delayed blowback guns otherwise the same AND IF OF EQUAL DEGREE OF DEVELOPMENT, RELIABILITY, AND VERSATILITY. Recoil has the track record now, and I would go with it in the real world.

January 20, 2005, 02:37 AM
the 3 viable blowback combat pistols in a "reasonable calibre (9mm)" that come to mind are the astra 600, the detonics pocket 9 and the AMT backup (in 9mm and .45)...none of these would be what i would call especially "user friendly"

in the "delayed/retarded blowback" camp, my personal favorites are both from HK...the P7 and the P9S. besides the novelty, both of these are excellent, accurate, reliable and easy to handle pieces.

January 20, 2005, 06:22 AM
I once did a test with three 380 pistols ,CZ-24 [locked breech], Remington 51[delayed blowback] and Mauser Hsc[blowback]. They are the same weight and size. The most recoil -the blowback, the least recoil -the delayed blowback. The HK P7 is a good example of a delayed blowback and those who first shoot it remark that the 9mm P7 recoils like a 380.

January 20, 2005, 08:47 AM
Thanks for the assistance guys… and thanks Jim for the valuable information you provided on both my threads.

Unfortunately, I'm still a bit unclear on the advantage a locked breech holds over a delayed blowback.

Say a handgun was introduced that had a roller under spring pressure underneath the chamber that engaged the slide. The slide would have to overcome the spring tension of this roller before: A) pushing the roller down and B) beginning its rearward reciprocation. I’m currently under the impression this would this be considered a delayed blowback. Am I correct?

Also, if this model was also offered with a locking breech, would there be inherent differences (strengths and weaknesses) between the two models?

Here are my suspicions (anyone's free to correct me if I'm mistaken):

1) The blowback might be inclined towards better accuracy as its barrel is in a fixed stationary position during slide reciprocation.

2) The delayed blowback may* yield slightly higher velocity with the same load as it has a barrel 1/4" to 1/2" longer than the locked breech with an equally long slide. * The reason I am unsure of this is due to any velocity gains that might occur from the cartridge wall/chamber seal during the dwell travel of the locked breech mechanism.

3) Perceived recoil would be comparable between the two.

If these above suspicions are for the most part true, why aren't there more delayed blowbacks on the market? I'm sure there's a simple reason that I haven't realized yet.

Thanks for the help.

January 20, 2005, 08:59 AM
If these above suspicions are for the most part true, why aren't there more delayed blowbacks on the market? I'm sure there's a simple reason that I haven't realized yet.

Simply put, economy. Complexity adds cost and you can't build a delayed blowback as cheaply as a straight blowback. Not only that the class of cartridges appropriate for a straight blowback are limited to pretty narrow bands of performance. On the other hand, no locked breech system other than the Colt/Browning or derivatives thereof is as efficient or economical to produce. That's why the locked breeches all work alike, except for the few that have the Walther tipping block. YMMV

January 20, 2005, 10:06 AM
1- fixed barrel always more accurate, 2- There are many different delayed blowback designs , some with different length barrels and some without.Bottle neck rifle delayed blowback tends to stretch cases a bit. 3 - Perceived recoil of delayed blowback is always less .With a Colt/Browning type action almost all the recoil comes when the slide goes all the way back and slams into the frame. It is the suddeness that makes it seem higher recoil. With the delayed blowback the recoil energy is spread out with time so it seems less. Measured recoil energy would be the same in two guns of the same cartridge and weight but perceived recoil energy would be different because the delayed blowback spreads it out in time.

Jim Watson
January 20, 2005, 11:11 AM
Why is up not down?
I think the answer as to why most medium calibre service pistols are recoil operated instead of delayed blowback is due to marketing and manufacturing trends over many years, not the operating system of the guns. I am not going for a PhD in Industrial Management and don't want to research a thesis but the short answer is:

It's YOUR fault.

You didn't buy enough delayed blowback guns when they were available, to support the market and factories.
You had the chance. Do you have a H&K P7 or P9S, Steyr PO18, Steyr Rogak knockoff, that Chinese thing with the P7 gas cylinder and Lignose Einhand triggerguard, Sokolovsky Matchmaster, NCG Gas Gun on 1911 frame, Benelli B76, Korriphilia PSP, Wolf Ultramatic, or Hogue Avenger? Did Dad and Grandpa buy a 1905 Mannlicher, Remington PA51, Remington PA53 (Ok, so there is only one of these.) Savage 1907, or Mann?

Some did, but not enough. The recoil guns got in first and stayed in. Theoretical advantages (IF present) did not outweigh Colt's 70 year head start in business combined with John Browning's genius.

January 20, 2005, 12:03 PM
I imagine there may be a stigma attached to the word "blowback", regardless if it's delayed or not. Sometimes just the fact a mechanism is unorthodox or at least uncommon is itself enough to denigrate the merits of such a mechanism.

Currently, I'm thinking if someone were planning to design and produce a 9mm or .40S&W pistol for the civilian market, they'd probably want to start off with a locking breech as it apparently garners the belief by most that it's the strongest, most efficient mechanism currently on the market (many likely feel this is why there are so few delayed blowbacks currently on the market).
Perhaps it is, though I remain unconvinced.

The just the word "blowback" seems to have accumulated a sigma so pervasive that I doubt a manufacturer could completely circumvent the general perception of the intrinsic inferiority of the delayed "blowback"... and perhaps it's just as well.
Over the past few months, I’ve become impressed with various locking mechanisms, especially the linkless locking cam. With the rear of the chamber pivoting down, both unlocking the action and (perhaps incidentally) facilitating in positive feeding from the magazine, combined with the chamber itself serving as the locking contact point for the slide, it truly appears beautifully efficient.

Oh well, just rambling on....

There is a reason for my seemingly repetitious questions; I've been tossing around a few ideas in my head for several months now that I might actually pursue. To that point, I thought it would be wise to retrieve knowledge from others first to thoroughly understand the task at hand.

It’s YOUR fault

Gee, it’s odd you should say that… I’ve been hearing that plenty around the house lately. :D

Anyway, that essentially concludes my volley of questions for now. Knowledge is power, after all.

Thanks again, everyone.

January 20, 2005, 12:25 PM
I imagine there may be a stigma attached to the word "blowback", regardless if it's delayed or not.

You may be on to something, especially since the common term was "retarded" blowback.

Jim Watson
January 20, 2005, 03:06 PM
If you think there is a stigma to the word "blowback" and if you have ideas involving unconventional breeching for an autoloader, include dreaming up a catchy name for the design.

I don't think the Wolf folks ever used the word "blowback" in describing their delayed opening system. But it didn't help them because the guns didn't work.

Yours will, right?

January 22, 2005, 02:15 AM
Here is an example I did for a .223


Subject: Re: blowback designs?

assume: Peak chamber pressure = 50kp/i/i
assume: average chamber pressure = 25kp/i/i
assume: Peak bullet velocity = 2500 f/s
assume: Barrel length = 16i = 1.33f
assume: brass case inside diameter = .35 i

calculate force from chamber = PA = [25kp/i][.35i/4][.35i/4][3.14]= 600p

calculate time of chamber force = 2 1.33f/[2500f/s]=.001 s

assume: action 2.5 i long = .208 f
assume: spring force = 20 p
calculate spring energy =fd=20 .208 =4.17 fp
calculate distance chamber pushes bolt = E/F =4.17fp/600 p = .0069 f
This means the bolt will be accelerating back for .001 seconds until it
has gone .0069 feet back and then it will be slowed down by the recoil
spring for 2.42 inches where it just runs out of speed as it reaches the
back of the action.
calculate bolt peak velocity = 2D/t = 2 .0069 f/.001s = 6.9 f/s
calculate mass of bolt = 2 E/VV = 2 4.17fp/ 6.9 f/s / 6.9 f/s = .175pss/f

calculate weight of bolt = GM= [32.2 f/s/s] [.175pss/f] = 5.6 pounds

If you increase the recoil spring force, the bolt weight requirement
will go down.

Jim K
January 23, 2005, 01:23 AM
The best known blowbacks on the market now for powerful cartridges are the Hi-Point pistols. And the slides are massive and heavy.

The whole name of the game is that the cartridge case must be kept from backing out until the internal pressure drops enough so that the case doesn't blow out and let high pressure gas into the action.

That can be done by using the inertia of a heavy breechblock, or by using some kind of locking system. In general, the choice has been to use a locking system as it is less heavy and bulky and easier to operate than blowback or retarded blowback systems.

Some "delayed blowback" pistols are really locked breech; the Remington 51 and Benelli 76 are locked. Unlocking is done by allowing the case to back out a short distance (exposing only the thick case head) and using the momentum generated to unlock the slide and continue the cycle.

The old S&W .32 and .35 pistols built on the Clement system of a light breechblock and heavy spring worked, but they had to build in a means of disconnecting the breechblock from the spring or the guns could never have been loaded for the first shot by anyone of ordinary strength.

The "linkless" system is merely a matter of adapting the later Browning system (as used on the BHP) to a 1911 type. It simplifies things, but has no other advantage.

Gas delayed blowback (HK P7, Rogak, Steyr GB, etc.) is tricky to make work, since the bullet has to be given a head start to clear the gas ports before any delay can take place. Like the Benelli system, it cannot allow the case to move too far back. The Rogak has no piston; the gas is simply directed toward the front of the slide. The trouble is that it takes too long and allows the case to move and start slide movement before the delay can take place and while pressure is still high. They could not stop the extractor from tearing the rims off the cases, so they took appropriate action - they ground off the extractor hook.

The fact is that your roller delayed blowback would probably work OK; you just have to get capital to build it and then convince the gun-buying public that it is a "better mousetrap". Note that the CZ52 pistol is NOT a roller delayed blowback; it is a roller LOCKED breech.

Incidentally, those exotic systems like the S&W, Remington-Pedersen, and Savage, all came about because of one simple problem. Browning had tied up every reasonable operating system in patents; one patent was for a breechblock as part of or solidly attached to a slide. That took care of the best ideas right there, so Pedersen, Searle, and company were off to the races trying to get around that big barrier.


P.S. I think someone calculated that a .30-'06 blowback would require a 13 pound bolt. Most SMGs get away with lighter bolts because they operate from an open bolt and can use advance primer ignition to use forward momentum to resist the blowback.


January 23, 2005, 05:26 AM
HK P9 is roller delayed, P7 gas delayed ,Remington 51 momentum block delayed. The guns that are in general called delayed blowback are usually called something else by their makers so that term is not a problem . The problem is that people find guns that are 'different' are hard to accept.I have a P7,Benelli M1 90 and had a M51 - all fine reliable guns.

January 24, 2005, 12:14 PM
Keenan, you're a helpful cornucopia of information as always, thanks!
And thanks for the clarification, Mete.

I'm not considering a roller delayed blowback; I just used that description to help myself understand the difference between it and a blowback.

What I have on paper is definitely a linkless Browning type lock as I can't find a more efficient way... one that's convincing anyway.
While flipping through a magazine, I found to my dismay that my design's breech lock (the 11th edition over the course of 7 months) which I thought was fairly clever was nearly identical to Ruger's new P345, with the means for the barrel to unlock from the slide on the rear of the recoil assembly... so it is no longer so, hopefully sufficient enough to steer it clear of any patent infringements. There are a few properties that I'm convinced will appeal to many of the members here, but those will likely become patented, so it's hush until then.

I can only hope the linkless browning cam is no longer under patent protection.

Thanks again.

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