throat/bore dimensions for Ruger GP-100?

[scattershot]

I have been getting some tumbling with moderate .38 special lead boolit loads from my GP-100. I have measured the cylinder throats at .355", and suspect that may be the problem. The boolits are .360". I have no way to measure the bore, however. Does anyone have the nominal specs for the throats and bore?

[GP100man]

If the throats are .355 they need to be reamed to 3575 then polished to 358 for shooting Boolits.

How did ya measure the throats ???

Were they clean when measured ???

Are you using 38 cases ???

Boolit & load combo???

Crimping or no , roll crimp or taper ???

What speed are the Boolits flying at & at what distance do you notice tumbling ???

[scattershot]

I measured the throats by dropping measured 9mm and .38 caliber bullets through the chamber mouths. A .355 9mm jacketed bullet hung up in the cylinder, and that's the dimension I figured the throats to be. The cylinder was clean, and the ammo I noticed tumbling was .38 Special, with lead SWC 158 grain boolits. I was shooting up some old ammo and couldn't tell you which load was doing the tumbling, but all the loads were nominal .38 special velocity, 800 FPS or so. Moderate roll crimp.

I never noticed this before, and in fact I was getting very good groups with this pistol initially. I suspect it may be the particular ammo I was shooting that day, but the chamber mouths being undersized made me think it could be something else.

Just had a thought, a few years ago I loaded some Lyman 358156 boolits without the gas check. Could that cause tumbling? This was at short range, 7 - 10 yards.

[scattershot]

Well, it's a mystery. I just got back from the range, and put about 125 rounds through the thing, and got 125 nice round holes. I used .357 mag JHPs, .38 special +p JHPs, and handloads at about 800 fps with158gr swc real boolits. No problems with any of them.

Go figure!?

[GP100man]

OK then , first push the 9mm bullet on thru & inspect it under magnification for one sided witness marks (oblong can also be measured with a MIC)

Second ,was the loads ya shot today with boolits you cast & were the others commercial??

[scattershot]

Yesterdays boolits were commercial, from Bull -X. Maybe it was the old loads without the gas checks that were the problem, but I'll check for witness marks as you suggested.

Thanks for the help.