Cocking Force: Kahr Vs. Original Full Auto

[reconbob]

In the course of some work here I just measured the force required to cock

a Kahr Arms semi-auto Thompson. Anyone who has ever handled one of these

knows that they are very difficult to cock. In a previous thread we kicked around

the concept of advanced primer ignition - where the forward movement of the

bolt dulls the recoil impulse of the cartridge because the cartridge fires a split

second before the bolt slams shut. Not everyone believes in this but if you are

a believer it allows for lighter bolts and recoil springs.

The semi-autos must fire from a closed bolt, so the dulling effect of the

forward moving bolt is not available and so the assumption is that stronger

recoil springs are required to make up the difference. Perhaps the standard

springs are heavier than they need to be because "easy pull" spring kits are

available.

Anyway the force required to cock a Kahr is 30-32 pounds. A lesser force -

maybe 28 pounds is all that is needed to compress the recoil springs but the

extra 2-4 pounds is needed to get the firing pin over the sear.

A standard full auto Thompson with bronze lock requires only 13-15 pounds

to bring it to full cock.

Eventually I plan to make a special receiver to enable measuring the true

recoil force of the bolt assembly to be able to answer the question of recoil

between M1921, M1928, and M1, bronze lock vs. no bronze lock, etc.

 

Bob

[Embalmer]

I just installed deerslayers ez pull springs in my kahr and i can cock it with my pinky. also since i installed it in febuary put 1550 rounds through without one jam or misfire. just an easier solution.

[Joe H]

Hi Bob,

 

I checked the cocking force on the semi's I have built. They were built with your recievers (a 1928a1 & Colt 21) and include working blish locks and oiler. With the rifle cocked there is about 5 - 7# on the closed bolt. The maximum cocking force is about 17-19# as the striker goes over and catches the sear. After the bolt is cocked it takes about 13 -14 # to hold the bolt full open against the return springs.

 

Joe

Edited May 14, 2010 by Joe H

[fortyfivecal]

thats good data guys, thanks for posting.

[45cal]

Are the ez pull springs still made?

[jim c 351]

Bob,

I don't own a semi, but going from the pictures I have, the receiver on the semi is much thicker at the rear, which reduces bolt travel.

Also I believe the bolt has about 1/8 " less height .

So ---lighter bolt--less travel-- I guess the semi would need stronger springs.

Jim C

PS the way the semi mashes primers I think they could get by with a weaker firing pin spring.

[nfafan]

Bob,

I don't own a semi, but going from the pictures I have, the receiver on the semi is much thicker at the rear, which reduces bolt travel.

Also I believe the bolt has about 1/8 " less height .

So ---lighter bolt--less travel-- I guess the semi would need stronger springs.

Jim C

PS the way the semi mashes primers I think they could get by with a weaker firing pin spring.

 

My 1927A1 with EZ Pulls needs a tad more than a pinky to charge, and it will still pierce an occasional primer. Still - tons better than stock and now makes me consider moving to an "acorn" charging handle.

 

 

Also; I noticed from my own semi 1927, and NFA 1928 and Reising M50s that I've fired; that these guns - despite their heft - seem to have more felt recoil than does my smallish MAC M-10/45. Maybe my little buzzgun is so fast that recoil is lessened(?).

 

I noticed a similar strangeness in a shoot-off between the Uzi, SWD M11/9 "MAC", and the omigoditsa MP5. The speedy M11/9 was the most comfortable of the 3 and - seemed the most fun, followed by the Uzi. The MP5 was kinda, meh.

 

Maybe it is that API firing phenom that was creating that impression...

Edited July 1, 2010 by nfafan

[reconbob]

Its interesting that you say that the recoil you feel from the MAC-10 is less

than what you feel from the Thompsons and the Reising. From a strict number

crunching point of view what you say would seem to be impossible. What does

a MAC-10 weigh? 1/3 the weight of a Thompson? The guns are firing the same

bullet. The weight of the gun affects the recoil impulse. A heavier gun will recoil

(move in the opposite direction of the bullet) more slowly than a lighter gun.

However, to you the MAC is more comfortable to shoot. I wonder why?

It could be any number of things.

I am reminded of the baseball cliche that a

baseball picks up speed when it skips on wet grass as opposed to dry

grass. Well, of course it doesn't pick up speed - that would be impossible -

perpetual motion, etc. What is happening is that the wet grass

slows it down less than the dry grass.

So what does a MAC-10 do that a Thompson does not?

 

Bob

Edited July 1, 2010 by reconbob

[Shattered]

However, to you the MAC is more comfortable to shoot. I wonder why?

So what does a MAC-10 do that a Thompson does not?

 

Bob

 

 

It's the shorter barrel.

 

Same effect is noticed in the HK series gun. Switch the G-3 to happy mode and hang on! However, you can easily and accurately hold an HK-51 (8.9 inch barrel) or an HK-51K (4.something inch barrel) on target with not much of a problem.

 

Only difference is shorter barrels.

[james m]

The H&K MP5K is among the most popular sub machine guns at our club. It is particularly popular with the female shooters due primarily to the perceived lack of both climb and recoil. I attribute this to the closed bolt operation along with the short barrel.

As an aside: If anyone is going to be in the Scottsdale,AZ area on the 4th of July the Scottsdale Gun Club is offering free machine gun rentals.

Jim

[nfafan]

I just cant explain why, but the sensation of firing the MAC-10 in semi (for apples/apples with the 1927) is that the MAC just doesn't "thump" the shoulder as hard as the 1927.

 

Maybe its the full mass of the bolt riding forward that causes some reflex that lessens the feel somewhat - as opposed to the "sudden shock" of a closed-bolt 1927 going off.

 

IIRC, the Reising M50 was a closed bolt, correct?

 

I actually fired a Reising before I ever fired a 1928 or my own 1927. I was stunned at the "pounding" - again, as opposd to what I was used to with the MAC-10/45. I was expecting almost zero "feel" given the heft and size of the M50.

 

Also, the MP5 was simply "ok"; didn't cause my blood to race just because it was an HK nor to get envious of never owning one. I thought the little SWD M11/9 and Uzi were more fun, and from that experience - I'd definately take an Uzi anyday over an MP5.