GP 100 Cylinder Throats and Bullet Sizing/Leading

[Thomas Traddles]

I have a GP 100. Its cylinder throats measure .3575 according to my pin gauges. I shoot 38 special loads in it --148 grain DEWC sized at .357. They are made from 20 to 1 alloy. I get some leading in the barrel, which affects accuracy and I think the leading is caused by the bullet being a wee bit small, not obturating enough to seal the bore when fired. Would there be problem with trying bullets sized .358? I've hesitated to do this because the bullet would be slightly larger than the throats. I worry about pressure build up from the bullet being required to be squeezed through the throat before entering the forcing cone. Any advice would be much appreciated.

T

[btroj]

I shoot .359 in mine. No leading, no problems.

[Echd]

My GP100 also has fairly tight throats. I shoot unsized fairly regularly from several molds (.359-0.360) with no problems and good accuracy.

The actual pressure required to swage down a bullet, or in this case even a boolit, is very small. You do it by hand with a loading press all the time anyway.

[Thomas Traddles]

Thank you, gentlemen. I will give it a shot, then, and see how things work out.

[c1skout]

Mine has tight chambers too, If I try any larger than .358 I have to SHOVE them into the cylinder.

[huntrick64]

I would start with a .358-.359 boolit to see what happens because that is the easiest thing to try first. Beyond that, "where is the lead" is the million dollar question. If it is immediately following the forcing cone, then the bullet is too small. This can obviously be because the boolit started out too small or was made that way by your tight throats or a constriction in the thread area. Either or both of these conditions can be fixed with simple procedures. If the lead is towards the muzzle, you are running out of lube or the lube is failing. I think most 357 cast shooters shoot .358 - .359 boolits. There are some pretty good threads regarding the relationship between the throats, the cone, the breach, and then the muzzle. Just keep in mind that if anywhere in the boolit's journey the path gets bigger than the previous spot, leading will develop just beyond that spot. Incorrect alloy hardness can also cause leading, but my experience has been that a properly dimensioned revolver/boolit can shoot some pretty soft alloys under a lot of pressure without leading.

[Bonz]

I normally shoot .358 in my GP100 with no leading

[captaint]

Mine pin guaged out at a very nice .358+. Made me very happy. Hasn't leaded up at all. Shoots very well. Mike