View Full Version : recoil springs and blowback actions

April 17, 2008, 08:11 PM
These questions are a primer for more discussion.

Comparing a standard pressure 9mm 115 Gr. to a .40 S&W 180 Gr.

Using simular actions, same bolt weights and other unaccounted items (Otherwise identical guns), the .40 would benifit from a heavier recoil spring, yes?

If the spring rates were the same would a heavier bolt help but increase muzzle flip? What effects would a lighter bolt have?

(For purposes of understanding: "benefit" would mean less wear and tear on the gun and shooter, all things being equall.)

April 18, 2008, 08:23 AM
Let's have another understanding: for a bullet of a given weight driven at a given speed, the recoil impluse is the same, regardless of firearms weight or the balance of the components therein. F=MA.

Now, the recoil impluse is not the same as felt recoil. The heavier the platform, the more intertia, and the less accelleration results. A=F/M.

For a given weight of firearm, the more mass that can resist (via its inertia) the recoil impluse, the less the felt recoil. In recoil-operated handguns, that means keeping the slide and barrel working together as long as possible. Julian Hatcher has a very useful discussion of this topic in Hatcher's Notebook, to which I commend your attention. The Savage M1907 .45 ACP competitor to the Colt M1909/M1909 Special (M1910) was felt to recoil much more savagely (sorry, couldn't resist) than the Colt, because the Savage incorporated essentially a hesitation lock; with the barrel and slide unlocked, there was less mass resisting the recoil impluse. The Savage was considered to be very unpleasant to shoot.

The one element in the 1911-type pistol that you can manipulate to alter perceived recoil is the time of barrel lockup. In the 1911 barrel lockup is controlled by the shape of the cam at the front of the barrel feet. That "cam " can be short, in which case the barrel unlocks quickly, or it can be longer, in which case the barrel unlocks later as the slide moves back. Later = more mass resisting the recoil impluse.

The relationship among slide mass, recoil spring rate and barrel unlocking rate is complex. It's a bit surprising to realize how well modern pistols work with a relatively wide variety of ammunition. If the relationship is "wrong," the pistol won't function properly. Thus, the parameter values you mention are generally set by the pistol designer for proper function with a given class of ammo (there's that recol impluse thing again) and can be manipulated generally only within a narrow range. Change the ammo, of course, and the relationships change. A 1911, designed to function with 230-gr "hardball," may not function at all with 185-grain "softball" target loads. Thus, a weaker recoil spring and short cam will usually be necessary. Plus P ammunition would indicate that you'd have to go the other direction.

You've raised some interesting questions. I wish I had more time to delve further into the subject, but I don't or I won't get my 1911 book written on schedule. Just keep in mind that F=MA is alive and well :) .



April 18, 2008, 07:26 PM
Thanks for the good reply.

I'm afraid it's not quite enough for me to do the math. In order to determine at least average acceleration I would need to know the unit of time it takes for the mentioned bullets to travel the length of a given barrel.

I'm asking because I'm trying to figure out why my CX4 carbine in .40 has a lot of felt recoil when almost every mention of it in 9mm or .45 guise states that their's was light or unnoticable.:confused:

I was thinking that it was design issues between the caliber and rifle. All three calibers use the same recoil spring after all. (There's a really long story about my quest for answers on this.)

James K
April 18, 2008, 09:00 PM
I assume the action is blowback, or are you using that term in a general sense?

In a blowback action, the return spring has no bearing on keeping the breech closed, that is a function of the mass of the breechblock. The spring can only slow the breechblock once it starts moving and, in an auto action, return it to battery. (A properly balanced blowback action doesn't need a return spring; for example, the return spring on a Ruger .22 auto can be removed and the gun will work perfectly, only needing the bolt pushed back after firing.)

It would be my thinking that the maker, trying to save money, used the same weight of breechblock for the .40 as for the 9mm. That would mean the former would exert more force, throwing the breechblock back harder and increasing felt recoil. Very likely the .45 version uses a heavier breechblock or even a larger receiver, keeping felt recoil down.

Blowback actions work differently from recoil operated actions like the M1911, and the rules aren't really the same. An M1911 pistol slide weighs a lot less than is needed for a straight blowback .45 like the Hi Point.


April 19, 2008, 07:08 AM
Through experimentaion I'm sure I could figure out if it was a flawed design or not, however. Realizing there's little i could do except use weaker loads than a standard 180 Gr. I'm probably just going to trade in for a 9mm version.

The correction would be far more effort than I care to exert. Using a mill and lathe I could add some slugs of tungsten alloy (Mallory metal) to the bolt for increased weight. The bolt wraps around the barrel and extends several inches forward of the breech which would be the ideal location.

I'll make some measurements before trading but I'm certain the only differences between the three calibers are the barrel and bolt. (Which cost almost as much as a new gun so converting is out of the question.)