Blowback with tapered/bottlenecked rounds?

[EMUGOD]

How does the blowback action (straight or delayed/locked) work with tapered and/or bottlenecked cartridges? Wouldn't the taper immediately break the seal? Are there other efforts taken to correct for this (there aren't that I can see), or am I overestimating the problem?

Sort of related, how would a length of smooth barrel before the rifling (at the proper land, not groove, dimension) work out? Would it have any effect on accuracy, wear, etc.?

 

[Jim Watson]

Early Bergman pistols shot bottleneck and tapered cartridges from blowback actions.
There have been a lot of blowback submachineguns in bottleneck calibers.
North American makes blowback pocket pistols in their proprietary .32 NAA cartridge which is a .380 necked down to .32.
The FN Five seveN is a delayed blowback with a long skinny bottleneck. Reloaders are warned about its headspacing requirements.
So it can be made to work.

a length of smooth barrel before the rifling (at the proper land, not groove, dimension)

It would not work at all. It would size the bullet down to land diameter so it would not engage the rifling and would allow blowby of powder gas.
A smooth section at groove diameter is commonly known as freebore and a poster here reported great results from it in .45 ACP. It has always worked for Weatherby rifles.

 

[EMUGOD]

So it can be made to work.

I know there are plenty that do it, but I am still curious as to how. I have been looking over various blowback designs and not seen any special methods to overcome it, so I am probably overestimating either the loss of backwards energy (once the powder has finished burning) or the effects of some of the gas escaping.

It would not work at all. It would size the bullet down to land diameter so it would not engage the rifling and would allow blowby of powder gas.
A smooth section at groove diameter is commonly known as freebore and a poster here reported great results from it in .45 ACP. It has always worked for Weatherby rifles.

Whoops, I guess I meant the land diameter then. I knew one or the other would not work, and made a guess.

Thanks for the proper name, but now I'm curious what you meant by 'great results', do you mean with standard rounds (with the bullet seated normally as opposed to farther back)? I am worried that too long a freebore (longer than the length of the bullet, which is exactly what I am considering) will cause all sorts of problems.

 

[Jim K]

The whole idea of a blowback action is that the mass of the breech keeps the cartridge from moving until the bullet has left the barrel and internal pressure has dropped to a manageable level. So the idea that a bottle neck or tapered round would "blow out" or expand as it is pushed back isn't the case.

If the round is too powerful for a blowback, not only would a tapered or bottleneck case expand, but the rear of any case would blow out as support is removed with the pressure still too high. That is why more powerful cartridges usually require a locked breech action.

There is some "fudge" there. With a very heavy breechblock (as in some pistols and in most submachine guns) blowback can be used with cartridges like the 9mm P and .45 ACP, but it was once calculated that a .30-'06 blowback would require a 26 pound breechblock.

Jim

 

[Jim K]

On the issue of a long leade (or throat or freebore), there is a fine line. Some freebore is needed to deal with different bullets in commercial loads. Military rifles, where production of both rifle and ammunition can be rigidly controlled, can be made with just enough leade for the ogive of the bullet.

But if the leade is too great, the bullet can move some distance and build up momentum before engaging the rifling. When that happens, the bullet "skids" on contacting the rifling, leaving markings that can affect accuracy. If the leade is very large, the bullet can actually yaw at that point and enter the rifling at an angle. The result is a distorted bullet and inaccuracy. That is exactly what happens when erosion eats away at the start of the rifling, and why target shooters are concerned about preventing (as much as possible) throat erosion.

Jim

 

[EMUGOD]

it was once calculated that a .30-'06 blowback would require a 26 pound breechblock.

Any source on that? I've been looking for exactly that formula.

 

[RedAlert]

I'm betting that the formula will require info that would be extremely difficult to obtain. If you get it, I'd be interested in seeing the data you need to supply to make that calculation.

 

[EMUGOD]

I'm betting that the formula will require info that would be extremely difficult to obtain. If you get it, I'd be interested in seeing the data you need to supply to make that calculation.

Why should it? It wouldn't be a function of maybe powder burn speed, bullet mass, cartridge mass, and bore (maybe also cartridge base diameter)?
edit: or, more simply, of the barrel pressure (and maybe cartridge mass).