adventures with a 1-13 twist slide mount

as somewhat of a followup to Jon's Thread on a KKM  1-13 twist Frame mount, I decided to build a 1-13 KKM wad gun using the typical slide mount. Specs for the gun follow:

1-13 twist KKM barrel

10 lb ISMI recoil spring

modified slide to lighten it up( see pictures). copied from a slide Jon did a while back that he showed me..

7 round Metalform Magazine...round follower and a wolff 6lb mag spring

Eulette Precision rail(lightest around)

aimpoint T1 copy with graphite tubes

All loads used Win LPP


This is a new build 

I'm a big fan of WST at the short line due to it's recoil impulse so my initial testing used it. My standard load for a flat base 180 Brazos or Summers bullet is 4.0 of WST..  For the testing I started with 3.8, 3.6 and 3.4 of WST to see where I stood. in my initial test 3.8 functioned fine but 3.6 did not cycle the slide fully. I then fired around 100 rounds of 4.0 to let the pistol settle in as it was pretty tight.

before my second day of test it dawned on me that my normal crimp for lead bullets is .468 so I decided to take that down to .465 for todays' test.. this time out 3.6 functioned perfectly as did 3.4..   no test targets as I was simply checking for functionality.. my indoor range is limited to 25 yards so no long line tests but all rounds seemed on call.  will eventually get the RR out and see how they run at the long line.

I will also develop some light BE loads just to give a base line with that powder.. I did have some 3.5 BE loads using a 200 GR. H&G 68 bullet laying around that functioned perfectly and actually were a bit snappy

1st picture is of my slide modification.
2nd picture is the rail set on top covering all the cuts.
3rd is the completed pistol

I firmly think the 13 twist is worth a look if you want reduced recoil..

many thanks to Jon for his help and yes I have stayed in a Holiday Inn before

Test with 3.2 of WST failed to fully eject, so for now 3.4 seems to be  the minimum of WST that works.with the 180s.

BE testing will be next using both 180 and 200 gr lead..

Eventually i may cut on the slide a bit more.

Crimp is worth powder. I see about 50 fps gain going from 0.470 to 463

I did go down to 463 on the 3.2 loads. Still no joy. Some things left to try but on to testing BE next.

Just curious, what weight is your mainspring and what does your firing pin stop look like? 

My ball gun (yeah I know, lighter slide) cycles as low 2.7 WST with 180gr swcfb, .465 crimp and 9lb recoil spring. I don't use the load, because it sprayed my arm with unburned powder particles and accuracy wasn't great with a 1:16 barrel but it cycled fine and locked the slide. I have a very rounded firing pin stop and lighter mainspring that were a big help cycling light loads.

Your results may vary of course because of the weight of the dot. My go-to short line load for lead is 2.7 gr vectan ba10 (similar to bullseye) and a 180 swcfb. Feels like a 38. And at the time when I stocked up, ba10 was the cheapest powder, 180s were the cheapest bullet... you may see a pattern. I can guilt free blast thousands of short line rounds a week in practice.

Currently has a fairly standard curve on my EGW FP stop and a 19lb mainspring which is my norm. 10lb recoil spring.

A 9lb recoil spring, 17lb mainspring and additional slide work are all possibilities. Not many rounds through the gun at this point so the unlocking is very tight. Accuracy at the long line verses recoil is the compromise we'll end up with.

Not having a 17lbs mainspring on hand, I simply cutoff 2 coils from a 19lb spring and tested a bit today, still with my 10lb recoil spring. I had no failures to fire with the new mainspring through 50 some odd rounds.

3.2 WST and a 180gr again failed to eject even though the pistol has noticeably less effort to unlock.
3.4 of WST continued to run fine..

Initial testing with BE:   .463 crimp

3.5, 3.3 and 3.1gr. with  180 Brazos all functioned perfectly including locking the slide back.

3.1 and 2.9 gr with a 200 H&G 200gr both functioned perfectly.

I'll keep lowering the loads with both 180 and 200 gr bullets..

I feel  2.9 of BE will work easily with the 180 just based on the recoil impulse of 3.1 today. we'll start there for the next test..

Recoil impulse between WST and BE is markedly different..3.4 of WST is quite mild while 3.1 of BE still has a bit of snap to it...

jglenn21,

I had to read this thread twice.  Do you have anything quantitative regarding accuracy?

Thanks.

Jim

nothing yet at the long line. all testing has been at my indoor range which is 25 yards..everything there has been on call. will eventually be testing at 50 with a RR. will test both the lowest load that functions against something a bit higher with both powders hopefully.

Hello jglenn21,

That's what makes life a horse race.  I'm not sure that I would have approached the process the same way but "you keep doing you".  Good luck.

Jim

Thanks, whole project is to simply see what the practical limits are for a slide mounted dot gun using powder puff loads verses a frame mount that Jon is working on.. honestly think we can get close but not quite what a frame mount can do..  hopefully generate some interest from others who work on their own guns

I, for one, am interested, as Jon is (hopefully) finishing up my 1:13 slide mount shortly. Saving me some load development!

Dave

Hello all,

I guess it all comes down to....what are the conditions where accuracy trumps recoil or vice versa?....

e. g. you have two different loads:
   Same accuracy but one has lower recoil - no brainer for me
   One loses 0.5" in accuracy but has lower recoil - personal choice
   One loses 1.5" in accuracy but has lower recoil - personal choice
   Etcetera
   One loses 6.0" in accuracy but has lower recoil - no brainer for me

Nonetheless, I remain interested in your results.

Jim

Dave be interesting to see how your testing compares to my slide mount. Every gun is a bit different...will hopefully.get more.testing done this week and probably shoot it next week in a match as my 45..

 I pretty much have always used a lighter load for the short line and a heavier load for the LL so i may end up with a heavier load for the LL and a very mild load that has acceptable SL accuracy..

One thing i have noticed so far is very small changes with slide weight really show up with these marginal loads.  I swapped dots last week and the lighter loads failed to function as they had the week previous. Both dots were T1 copies and appear very close to one another but the one was a tad heavier.. swapped back to the Primary Arms dot and continued to test. Heavier dot was a Sig..

Yeah, it might be different, but what you come up with will be a good starting point. Mine will have a Kodiak rail with an H1, trying to keep it light. I typically use one load for both lines for simplicity's sake.

Dave

not much time for load development or testing this week but did get out to the range with a buddy Tuesday and shot some more test loads.

with the 180gr Brazos  both 2.9 and 2.7 grs of BE functioned perfectly. The 2.7 load is beginning to feel a bit closer to the WST loads in recoil,  but BE is just pain snappier than WST..

with a H&G 68 200gr bullet both 2.7 and 2.5gr of BE work fine.

we'll keep lowering BE loads until function becomes an issue. May just drag the RR rest out next week if we get to practice some midweek.

going to shoot the pistol this weekend at a match using the lighter WST loads.

When Adam was young (about 10 years old) I made a 1911 for him. I did have a Colt National match in 38 special but he didn't like it. To lower weight I drilled holes in the bottom rear end of the slide, when removed it looks like Swiss cheese! To lower the pressure required to depress the disconnector I milled a ramp just wide enough to fit the disconnector that I put a slight taper on the sides. The is no bump now when the slide hits the disconnector. Problem I have with light recoil springs is where for accuracy the pressure needs to be the greatest spring pressure is lightest. 10 pound pressure the spring provides is at the location when the slide is back and decreases to preload pressure when slide is closed. To help the slide close I use what Floyd Aikman taught me to do on the lower lug ramp and don't allow the link to do anything to lift the barrel nor do I round the leading edge of the lower lug.There is no lock-up bump. When checking the 50 yard accuracy you might want to try that method if accuracy is poor. I also found it cures vertical stringing issues.

Froneck wrote:... To lower the pressure required to depress the disconnector I milled a ramp just wide enough to fit the disconnector that I put a slight taper on the sides. The is no bump now when the slide hits the disconnector....

Is that what is sometimes called the "Marvel cut?"

I don't know what it is called, I haven't looked into what others are doing. I simply noticed the "bump" and did what I though best to remove it so that the slide didn't have anything (though very little) to slow it down. It was interesting that the AMU was doing the same thing. So I'm not the "inventor" of the idea as it was probably done by others so I don't want credit for it but just want to pass it along to others. Marvel may have been the guy that came up with the idea possibly like me noticed the problem or might have seen it done by someone else. I simply don't know. I started working on my 1911 after having one made to BE quality but after a few matches and practices the group started to increase so back to the smith and a few more $$ it was good again but after a while it needed another tune-up! I started reading books on accurizing the 1911 and noticed I too was getting the same results and had to repeat the process. So I started looking at doing it dimensionally in that if everything is made to the correct dimension accuracy would result, the same way tool and die makers do it on production machinery. I was helped by Floyd Aikman, George Madore, Carl Mustoe (Carl graduated the Colorado School of Gunsmithing) and my father who knew nothing about the 1911 but was a very good tool and die maker.

SonOfAGun wrote:

Froneck wrote:... To lower the pressure required to depress the disconnector I milled a ramp just wide enough to fit the disconnector that I put a slight taper on the sides. The is no bump now when the slide hits the disconnector....

Is that what is sometimes called the "Marvel cut?"

Marvel used that term, but there are other pistol designs that came like that from the factory. Basically sloped disconnector rail.
It only helps when loading the first cartridge from the magazine; if disconnector and slide are correctly modified it’s not necessary.
When pistol fires the slide cycles to the rear and pushes the disconnector down. The slide returns forward into battery and the disconnector is still down because the trigger is still to the rear. When the trigger is released the disconnector will go back up into the disconnector notch in the slide and re-engage the sear for firing. 
So as described the disconnector only can be a speed bump during loading of the first cartridge. 
Most smiths add slight radius to the front of the disconnector/breech face corner and radius/shape the disconnector to allow smoother transition of the disconnector at that corner. Also another reason to shoot 4 rds and reload magazine during SF strings.
The Marvel cut is grossly overrated.
Having said that, I do on occasion cut it into slides.
Jon

Jon is correct, I neglected to mention the "Bump" will only happen on the first closing of the slide. Not sure if someone can release the trigger soon enough but I guess it might be possible so that the disconnector will be up. However most shooter especially me hold the trigger back for a while after the slide has closed. About all that is done is to help the slide close the same way on the first shot as it does on the other 4.

Wow 2.9 grains pr 2.7 grains of Bullseye and a 180 grain bullet, that is very impressive if it functions and is reasonably accurate.  I thought my 3.7 grains of Bullseye and a 185 grain lead swc was a light load.  Thanks for telling us about this build.

I'm wondering if the 1-13 is available in gain twist?

Froneck wrote:I'm wondering if the 1-13 is available in gain twist?

Why would you try with such a short barrel? Schumann gain twist barrels never grouped very well in all the testing I and some others I know performed.
I know black powder rifles have shot phenomenal groups with them but they are 30+" barrels and can change the twist gradually vs immediately with 4" of rifling.
Jon

I'm pretty sure there was a run of S&W 52s that had the "marvel" cut.. but it's be a while