Model 638 Frame (Bore) Constriction Excessive

[The Bolt Man]

My new S&W 638. Test firing indicated accuracy was not up to par. I noticed when firing the 148 grain Berry plated HBWC, accuracy was terrible and bullet holes indicated the plating was peeling off. These Berry bullets have been working very well in S&W Model 14 - three tee's target and two 357 magnums with reduced loads. Recovered bullets show the plating was splitting along the rifling marks. The bullet skirt was not being blown open. Lead bullet ammo leaded the bore badly in the first inch of bore.I slugged the bore by firing a Hornady lead HBWC that starts out at .359" diameter by firing it with 1-1/2 grains of Unique into a box of compressed rags. The bullet simply bounced of the top of the rags. That bullet then measured .3554" using ring gauges.

I have been emailing with another forum member and he opened my eyes to to something I completely missed. My mind must have been clouded by my concern about the gun not doing well. He suggested I could have a condition called frame or bore constriction where the barrel screws into the frame. Then I slugged the muzzle end, first 1/2" of bore and that measured .3574+".

That means I have a .002" constriction where the barrel screws into the frame. This may be within S&W specifications but is a disaster to accuracy and the plated bullet can not withstand the pressure applied to the bullet by the undersized bore. Lead bullets lead the bore badly.

Assuming S&W may consider this constriction within specification or may rebarrel the gun and still have a constriction, I have decided to lap out the constriction. That will give me a pistol with little or no constriction and that should eliminate all the problems. Certainly better than factory.

These constrictions in revolvers are not a new thing. The thinner the barrel the worse the constriction.

 

[notsofast]

How will you lap the barrel? And will it be done full length? If not full length how will you only work the tight area/areas? How will you keep the bore round and how will it affect the "rifleing"?

 

[The Bolt Man]

Fire lapping is the process I use. This process uses a diamond dust in a grease. You have two steel plates that you apply the grease to, then you use a lead bullet of the appropriate hardness, roll the bullet between the plates applying pressure so the diamond dust is impregnated on to the surface of the bullet. The bullet is loaded over a very light powder charge so when fired the bullet will barely exit the barrel. Each pass of these bullets will remove a very tiny amount of bore metal.

When the bullet is in the right hardness range, it will remove metal from the tight area of the bore, in this case the frame constriction, and will be reduced in size by the constriction, therefore it can't remove metal from the rest of the barrel. A bullet too hard would, as the constriction is reduced, remove metal forward of the constriction, because a harder lead bullet will spring back to as much as .001" or more after passing through the tight area. A soft lead bullet like a swagged lead bullet will be slower removing the constriction but doesn't spring back enough to be a problem. It also works less fast because it applies less pressure on the barrel as it passes through the constriction. The suggested bullet hardness for best results is around BHN of 12.

You do have to slug the bore often enough to understand how many rounds are required to remove say .0005", something you can be sure of as a notable change. Dependent on the bullet hardness used this fire lapping to remove .002" constriction it could require between 50 to 100 rounds. It will be best to slug the barrel at number 25 to see at what rate metal is being removed and work from there.

 

[photoman]

The Model 638 is a fighting gun, not a 50 yard bullseye gun. I guess if you want to fret over two thousandths of an inch for the fun of it, then ok but it's sure not going to make a bit of difference if you're using the gun for it's designed purpose.

 

[earplug]

I have seen various pistols leave patterns of spun off plating residue on close range targets. Accuracy with plated bullets is problematic at best.
Your leading might be from undersized bullets or to hard a alloy that won't allow a good gas seal with the bullet base.
I know Charter Arms and I had a problem with a 44 SPL that had a constricted barrel joint. Soft lead worked for it.
I would go back to square one and try some factory 158 grain bullets and see what results you get. If your target shows signs of keyholing bullets, its time to call S&W.
My J frames perk along just fine with a soft alloy bullet that is .358-360 depending on the mold i'm using.

 

[The Bolt Man]

Photoman44,

Your opinion and you are entitled to it.

Me, when a gun shoots like craap and I can fix it, I do. Otherwise I don't own it for long. MY OPINION.

earplug,

The same Berry plated bullet shoots very well in my S&W M14, Ruger 357 mag. Security Six and Ruger 357 mag. Vaquero with very good accuracy. The Security Six I had to fire lap the frame constriction out. None of the three guns does any harm to the plated bullets. Plated bullets simply do not like being squeezed to death then have to expand back up to fill the remaining bore.

At first I wondered if my load had been over powering the hollow base as the bullet exited the muzzle and was blowing the bullet skirts open, but none of the recovered bullets indicated that.

That points to the 638 as being the problem because of the .002" constriction. I fully expect to see the problem go away when the constriction is gone.

The Lead bullets I fired were Hornady 148 grain swagged HBWC. So they were not too hard, in fact very soft and they measure .359" diameter which is plenty big enough to more than fill the bore even after going through the constriction due to the nature of HBWC bullets and the pressure that remains in the bore. Accuracy with ten rounds fired was very good, but additional shooting would have simply added more lead to the bore and accuracy would get worse. The leading started right where the constriction is. The constriction is the major part of the problem.

Gotta go. I have to start making up the firing lap bullets. I will come back with the results to test firing after I complete the fire lapping and test firing.