Barrel rifling width

[Julien Boit]

Hi all,

I have one of my pistol fitted with a KKM barrel, a .40sw, and I posted previously regarding accuracy problems with this pistol.

After some times, I thought my problem was solved as after some shooting the accuracy was Ok for my goal.

But this week end, I shot the pistol on a bench, and at 15 meters, I couldnt' get two rounds next each other.

The spread was huge and the "group" measured around 4 to 5 inches !

I took a hard look at the barrel this morning hoping I would spot something suspicious.

It seem to my eyes, that the width of the rifling is wider than in a Schuemann Ultimatch barrel I compared to.

Can it affect accuracy ?

The lock up of the barrel is very good, and the slide to frame fit is Ok too.

So, I can see nothing else but a barrel problem.

Help pleaaaaaaase

[HSMITH]

Is the group round or is the dispersion more horizontal or perhaps more vertical? What are the specifics of your load?

The KKM barrels I have in my 2011 pattern guns are extremely accurate, as accurate as any barrel I have. A typical pistol barrel is short and very rigid, even a poor barrel will shoot 3" at 25 yards if the fitting is decent. A quality barrel like the KKM will shoot way under that, usually into one hole slightly larger than the bullet if the shooter and load are capable.

I would suspect that either something is loose and moving on your gun or that the fitting of the barrel is not right. Information on the dispersion of rounds in the group will help.

[Julien Boit]

The load is the one that I currently use in my sight trackers (OEM SVI). It's very accurate in my two SV, have been very accurate in several STI as well. The load consists of Fiocchi 170 FMJ loaded to 1.20 OAL. 5.5 grains of Vihta N320 and Fiocchi SP primers.

The group is erratic, the spread is not consistent. First shot can be high right, next low left, then high left etc...

I've had a couple of KKM barreled pistols in the past, and they proved to be very accurate, but this time..........

[HSMITH]

Load sounds good.

Would you say the dispersion tends to be more up and down or side to side? Or is the 'group' more or less round? How far can you retract the slide (exactly how far) before the barrel starts to drop down?

[Julien Boit]

I would say that the group is round, very erratic, but looks round.

Gotta measure the slide travel before unlocking.

[HSMITH]

While you are measuring things measure upper locking lug engagement with the gun together and then with the top end off and just finger pressure locking the barrel up. Any chance it is a bushing barrel?

[Toolguy]

If it's an open gun, the scope could be loose or damaged on the inside. Then try a different scope for a test.

[Julien Boit]

Ok, did the measurements.

First of all, this is a standard class pistol, not an open gun.

The slide has been flat toped, so the height from the bottom of the rail to the top is .966 inches (24.54mm).

With the barrel hold by finger pressure, from top of the slide to barrel hood : 0.055 inches (1.42mm).

With the pistol assembled : 0.059 inches. (1.5mm).

And finally, this is a bull barrel.

The slide travels approximately 0.153 inch (3.9mm) before the barrel starts to get down.

Edited September 1, 2008 by Julien Boit

[HSMITH]

Sounds like the upper lugs aren't fully engaging if you get a shorter dimension with finger pressure than with the gun assembled, at least a part of the variation is in the rear of the barrel not being held tightly during firing. The lower lugs will need to be welded up and re-cut to fix this so that the rear of the barrel is locked between the upper lugs and lower lugs preventing movement on firing. Also measure how much upper lug engagement there is and post that. The slide travel is a hair long but it should be fine, but combined with the slack in lock up I would think the link is not the optimum size as well.

[Radical Precision Designs]

Hi, Julien.

Most of the recent manufacture of KKM bull barrels are made way oversize and require quite some fitting. But I like them that way, as it allows me to tune-fit them real nice and tight. The problem usually is that many customers (including some 'smiths) expect them to drop right in with just minimal fitting at the hood and feet.

Use a black magic marker or machinists Dykem blue/red and coat the barrel up externally. Remove the guide rod and recoil spring from your gun, and then assemble the slide and barrel assembly (no lube on, please) to your frame. Now with some fast hard motions cycle your slide back and forth a few times. Disassemble the slide and remove the barrel. Observe for "witness" rub marks in the areas of engagement to your slide. It will show where additional fitting will be required. Oftenly it will show marks on the sides where the grooves start showing contact that does not allow the barrel to seat completely upwards.

The biggest item very often overlooked is at the top front of the barrel where the rearwards taper starts. Since the barrel is made so that it can be shortened to a commander length, you will notice that what we usually would refer to as the "belling" in the barrel is quite far back. You will notice eliptical and sometimes side rub marks where it contacts the slide on full up engagement. Unfortunately this will cause the barrel to exert binding pressure on itself which will create a pattern situation such as you have noted. The cure is to move the upper and side relief of the bull barrel "belling" to just around .250" (1/4 inch) behind the muzzle. This will allow the barrel to not bind out of lock, and if the upper lugs are properly relieved to match your barrel link, then your are good to go. You can use a fine flat file or a narrow belt sander to accomplish this. Do it slowly, checking twice, etc, and finish by polishing the file marks ...

Hope this helps ????

[Julien Boit]

I'll check this Venry, thanks

[Julien Boit]

I checked what you said Venry and Indeed, there were rub marks.

I removed these contact area and I must say the locking / unlocking is smoother than before.

I'll check the accuracy this week end.

I'll let you know

Edited September 4, 2008 by Julien Boit

[want2race]

I just had this exact same thing. KKM. It was locking up on the center, instead of the muzzle. Lathe and paper is all it took to get just a hair of material off and the lock up is smooth.

Sounds like your barrel was in tension (leaf spring) while in lock up. The 1911 bible also covers this.

[HSMITH]

I am still troubled that the barrel will lock higher with finger pressure than it does on the slide stop, doesn't indicate a problem with the muzzle end fit and should indicated that it isn't sprung in lock up. Am I reading his checks right?

[Radical Precision Designs]

I am still troubled that the barrel will lock higher with finger pressure than it does on the slide stop, doesn't indicate a problem with the muzzle end fit and should indicated that it isn't sprung in lock up. Am I reading his checks right?

Howard.

Your reading does follow what Julien posted regarding the numbers included, although it is only a few thousands which could be accounted by the fact that the barrel was held in position without using the frame as a vertical positioning guide. As you know most slides are far from being completely symmetrical. My "guesstimation" was based on experience and acquaintance with a few barrels and their little manufacturing variations and installed behaviour. The real give-away was the round dispersion, which probably got worse as the gun warmed up. If anything (IMHO) is needed from the bottom to tidy up things, it would be a slightly different link to make up any difference for maximum lock up without binding or scraping, etc. I like my barrels to be solid without binding anywhere, especially as the parts heat up. Many people make the mistake of putting too much up contact on the link (not remedial) which changes the geometry detrimentally, or otherwise a "bump" spot in the camming area. This makes for a false solid lock, as the only spot of contact should be at the rear of the feet in conjunction with the stop pin. If this is done properly and there is no bind/lock spot in the middle of the barrel, then the harmonics will not be affected to cause the "round dispersion" pattern. It will also yield the observed benefit of smoothing up things by eliminating unnecessary friction spots. He could still have some dispersion if at lock up there is some concentric play at the muzzle between the barrel and slide.

Anyway, I have faith in Julien. He has been around the block a few times. ( I know he is hunting around to find some "old" francs for me ... none of those silly new euros !!!)

[HSMITH]

Thanks for the reply Venry, point understood. It will be interesting to see what finally shakes out.

[Julien Boit]

Venry, you're my hero !

The pistol is no more a shotgun !

I haven't tested it on a bench, but on a 20 meters target, there is no more lateral dispersion.

I will put it on a bench next week end.

I think I can dig up some old "francs"