Jump to content

Recommended Posts

Been doing alittle reading on Kahrs and watching videos on both semi and the real deal full autos and the griping about stiff springs. I'm kinda new to these things,but whats sticking in my head is how easily the open bolt guns cock versus the horrible scene of guys trying to chamber rounds in the semi's. When you see dismounting videos the open bolt guns have one spring, dose'nt look all that stiff, but the closed bolt semi's have two and appear to be way stiffer. I see where replacement springs kits are made to help with this, but the question remains why such hellish recoil springs on the semi's?

Link to comment
Share on other sites

I’ve fired a full auto Thompson years ago but don’t remember how hard or easy the bolt worked but a full auto Thompson is an open bolt gun so you never fight the bolt. It locks back when the mag is mty or you release the trigger. Pulling the trigger releases the bolt firing the gun. The semi is a closed bolt gun. You have to cycle the bolt to load.

Edited by Baltimoreed11754
Link to comment
Share on other sites

I'm not "the guy" to ask, but I'm pretty sure it has to do with NFA rules. More to the point, they had to make it as difficult as possible to convert the firearm to full auto as possible.

 

YMMV, Karl

Link to comment
Share on other sites

If I remember correctly, NFA rules that a semi auto has to be a close bolt system.

Since it doesn't have a locking lug system then I'm assuming the bolt needs to kept all the way forward to fire.. and can have a risk of bolt bounce. Installing a stronger spring can help stop the bolt bounce.. hence why I think there could be a possibility that the semi has a stronger spring than a full auto. Well that's my thought upon it.

Link to comment
Share on other sites

when you cock a kahr gun you are pulling back on the main recoil spring plus the firing pin spring and as you pull back the firing pin is draging over the sear and sear spring, a lot of friction, no wonder they feel like crap,, when i cock my 28 navy thompson it is just slick, big difference,, just sayn

Link to comment
Share on other sites

  • 6 months later...

The reduced spring and buffer kit from Howell sounds like a great idea. The only problem I see, is, in a straight blowback action, the springs are regulated to allow proper functioning and to slow down/buffer rearward travel of the bolt during firing. If the springs are lightened, the bolt will travel rearward with much more force than normal. I know the polyurethane buffer is designed to prevent damage, but......has anyone here used one of these kits for awhile and noted any damage to their gun? I have a newer Kahr, and am interested in the kit. Seven surgeries and residual neuropathy really makes operating my Thompson kinda awkward and painful! Cheers!

Link to comment
Share on other sites

Semi Auto Thompson fires from closed bolt. Uses a modified M1A1 Thompson bolt which has less weight than a regular M1A1 Thompson bolt.

Semi Auto uses 3 springs, two recoil springs plus a regular Thompson spring which powers the " hammer" which strikes the firing pin.

The 2 strong recoil springs make up for lighter bolt weight in semi auto.

In unlocked blow back action bolt weight and spring power are critical. The M1/M1A1 Thompsons and the semi Auto cousins are both unlocked blowbacks.

 

Have used light power springs and buffer in my semi auto Thompson for 3 years now, no evidence of battering.

The buffers need replacement every so often but no other issues. I use only regular ball ammo or "mild" reloads with FMJ RN bullets in mine.

 

Do a search here on recoil springs for more info.

Link to comment
Share on other sites

  • 2 weeks later...

I could never find the EZ pull kits in stock, so I snipped a few coils off mine (one at a time) to make it easier and it worked, however...

CAUTION: if you remove too many coils the gun won't cycle correctly and you'll have to replace the springs altogether at that point.

Link to comment
Share on other sites

I could never find the EZ pull kits in stock, so I snipped a few coils off mine (one at a time) to make it easier and it worked, however...

CAUTION: if you remove too many coils the gun won't

cycle correctly and you'll have to replace the springs altogether at that point.

Whats the magic number? Dont leave us hanging.

Link to comment
Share on other sites

 

I could never find the EZ pull kits in stock, so I snipped a few coils off mine (one at a time) to make it easier and it worked, however...

CAUTION: if you remove too many coils the gun won't

cycle correctly and you'll have to replace the springs altogether at that point.

Whats the magic number? Dont leave us hanging.

lol, I honestly don't remember. I'd clip, shoot, clip shoot, repeat until the resistance on the charging handle was acceptable and then stopped there.

It's been several years ago, but I think it was somewhere around 3 or 4 coils?

Tray

Link to comment
Share on other sites

This is what I used. The spring force is lessened noticeably, but with enough force to prevent excessive impact of bolt at the rear of the receiver and feed reliably using a WWII era drum. A buffer was cut from polyurethane and holes bored to fit around the firing pin and recoil spring guides.

Purchase from McMaster Carr:

 

Springs - 9637K65 pack of 5 ea 11" long - cut to 10" $12.86

Buffer Material - 8789K32 polyurethane sheet 6"x6"x3/16" Dur 60A $15.61

Edited by DARIVS
Link to comment
Share on other sites

  • 4 months later...
  • 3 months later...

I just ordered a reduced spring kit. Their credit card processor will no longer accept your credit card. You have to call it in.

They've been dumped from CC processing on their Web site because they deal in fire arms.

I guess we'll see more and more of this crap.

 

The upside it that the company is great to deal with by phone.

Link to comment
Share on other sites

I've fixed a couple. What I have done in the past is clip coils as mentioned above and made an extended cocking handle. Pretty simple to do and greatly reduces the strain on the hand. What amazes me about these is they took a gun that uses one light recoil spring and turned it into three springs, 2 recoil and a striker. What should have been done is for the designer to realize that the striker spring becomes a recoil spring when the striker is in the fired position. I have an idea to make one that uses the original recoil spring and adds a simple striker system that is pretty close to internal to the bolt. It is on my to do list to fab a complete replacement system if I can ever get some help in the shop, :wacko: guess I need to start looking for one to buy to play with.I'll get to it one of these days :D

Edited by Mad Machinist
Link to comment
Share on other sites

  • 2 weeks later...

I like you're thinking MM, the answers that seem most plausible to me are to prevent bolt bounce. I can kinda visualize in a full auto the bounce not being critical because the bolt's going back anyways. It seems like the semi should have a heavier bolt than the full auto if it needs to remain shut for a few milliseconds. I'd like to see an oversized bolt knob made that mimics the original, just larger. I hope you get time to pursue your idea's.

Link to comment
Share on other sites

There're a number of factors used to determine the appropriate bolt weight, recoil spring rate, and recoil spring preload on the bolt. For detailed information look up AMC Pamphlet 706-260, February, 1970.

 

In any case, a heavier bolt handle has been tried. The handle would eventually break at the thinner section where it travels in the slot in the receiver.

 

Something that probably could be done to reduce Thompson 27A1 recoil spring effort, but would also probably be prohibitively expensive is to insert tungsten rods, disks, and/or plates in the bolt to increase the bolt weight (as is done, for example, in the mini Uzi submachine gun for rate of fire reduction); and reduce the recoil spring rate accordingly. As an aside, for the Ingram pattern ("MAC") M11/9 and M11-380 submachine guns, solid steel/tungsten alloy bolts and accompanying recoil springs have been made, specifically aimed at reducing rate of fire. The Ingram pattern steel/tungsten alloy bolts each weigh about twice their respective all steel, standard parts; and are quite expensive, even when newly made.

 

MHO, YMMV, etc. Be well.

Link to comment
Share on other sites

Didn’t the Polston design deal with this and other kinks in the original WH semi ? Problem was none of the Polsten guns were actually produced beyond the prototype stage AFAIK. Believe Doug Richardson was involved with the project at one point.
Link to comment
Share on other sites

Didn’t the Polston design deal with this and other kinks in the original WH semi ? Problem was none of the Polsten guns were actually produced beyond the prototype stage AFAIK. Believe Doug Richardson was involved with the project at one point.

The Polston design (embodied, depicted, and described in U. S. Patent No. 7,562,614) appears to be directed to a fire control group which, by itself, would not likely have any substantial effect on recoil forces. If the Polston design fire control group were to be implemented in a firearm that also implemented a type of delayed blow back, e.g., the Blish device system, the firearm would likely be able to implement a lower effort recoil spring assembly.

 

MHO, YMMV, etc. Be well.

 

ETA: FWIW, Polston also obtained a patent (U.S. Patent No 7,779,741) that is directed to improved ammo feeding in a firearm such as the Thompson.

Edited by Merry Ploughboy
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

×
×
  • Create New...