Slide Mass vs. Recoil Spring and Slide Velocity on BlowBack Actions

[keys85]

I read somewhere about somebody who converted a 9x18 blowback action (Makarov) to 9.2x19. Easy conversion, and makes use of 9mm luger cases to load for the pistol without trimming all the brass.

The longer case has a higher capacity and is essentially a 9mm case popping out a slighter fatter bullet. I began researching load data for 9.2x19. It appears some have been attempting to load this round hot, to the bottom end of 9mm luger levels. Of course, a gun chambered in 9x18 isn't designed to handle that.

I understand that in a blowback action the mass of the slide is overcome by the ejecting case. The only way to make a 9x18 gun shoot 9x19 is to significantly increase the mass of the slide.
Some, to cope with the hotter rounds, put much stronger recoil springs into the gun. I hear that this does not effect the function of the pistols intended design. Doesn't recoil spring weight play a part in the blowback recoil system?

Can somebody explain to me why you can't take a blowback action, put a significantly heavier recoil spring in the gun to compensate for slide mass, and shoot loads hotter than intended? Aside from overcoming the mass of the slide, doesn't the recoil spring lower slide velocity and increase resistance of the slide over duration?

Theoretically, I don't understand why if I replaced my standard 17lbs recoil spring to say a 25lbs recoil spring why I couldn't load up rounds in 9mm luger levels.

 

[T. O'Heir]

"...Doesn't recoil spring weight play a part in the blowback recoil system?..." Yep. Changing the return spring will make a difference.

 

[Kortik]

You can play with the spring and make it work for hot loads. But then it will not work reliably with not so hot loads. You know that. Another potential problem - I suspect some mismatch between magazine spring and recoil spring may cause slide to go into battery before full cartridge delivery
from the magazine takes place - you may have feed problem. Seems like increasing mass of a slide is preferable to address this potential issue.

 

[keys85]

Well the problem is not having a locking-breech. With hot loads, it's going to want to open the chamber sooner causing problems probably ranging from bulged cases to hot gas coming out the ejection port or worse - case head separations. But a stronger recoil spring would delay this, right? So...why is it said that 9x18 is the hottest you can go on a blowback action? With a very heavy recoil spring...I'm not saying it's possible to get 9mm luger performance, but at least the low end of it. I wonder why the Russians didn't beaf up their pistols a bit to handle it.

Eh...I pretty much answered my question and also did some supplementary reading. But I'm thinking, I wonder if 50-100ft/sec can be obtained safely with stronger recoil spring. I wonder where "the line" is drawn before you get an oopsies.

 

[Crosshair]

So...why is it said that 9x18 is the hottest you can go on a blowback action?

Because people don't know better. Hi-Point proved that you can chamber reliable blowbacks in 40 S&W and 45 Auto. You just have to have the gun be the size and weight of a brick. For a $180 gun it works rather well.

In small arms, relying on the relying on the recoil spring more than bolt mass requires careful tuning to get the weapon timing right. Something that some makers don't really bother to do.

The largest blowback operated weapon as far as I can find is the Oerlikon 20mm cannon.

 

[Jim Watson]

The Astra 400 (9mm Largo) and 600 (9mm Para) did OK for straight blowback service pistols in serious calibers. They were carefully balanced for slide mass, recoil spring strength (high) and mainspring resistance (substantial.)
I once read that the post-war Falcon with exposed hammer was less successful, with noticeably heavier felt recoil due to the lighter slide.

 

[Slopemeno]

Interesting topic. I recall reading in, I *think* Hatchers Notebook that he did some calculations on a blowback medium machinegun in .30-'06, and the bolt would have had to weigh around 18 lbs.