GP100 woes

[BullitHolz]

Hey guys, I have a GP100 that I picked up new about 8 months ago. I've put quite a few rounds through it, all jacketed, and it shoots 3" groups at 10 yards. I think most of that group size is me so I'm not to concerned about that. what does concern me is that, in the interest of saving cash I started loading lead to shoot in this gun and that is when the trouble started.

Simply stated this gun will lead the heck out of the barrel, badly enough that midrange loads(800 FPS or so) will keyhole targets with the first round. This whole thing got me really curious so I did a LOT of reading on the subject. Well pretty much everyone in the know suspects that these Rugers have cylinder throats that are too tight and often need to be reamed to .3585 to shoot lead with minimal effects. So I decided to check the throats on my gun and find out that my problem is the exact opposite, the throats are oversize. I slugged the throats and then mic'ed(not calipers) the slugs and noted that 4 of the throats are .3595 and 2 are .3605. Needless to say bullets drop right through them.

I debating about sending this gun back to Ruger but I might just keep it and only shoot jacketed rounds in it.

Has anyone else seen this and is this common with these guns?

 

[rcmodel]

I suspect you are buying bevel-base hard cast bullets.

They are too hard to slug up to fit the over-size throats at mid-range pressure.
And the bevel base contributes to gas blow-by.

The cast bullet companies prefer harder alloy then normal, and bevel-base molds, because the auto-casting machines run much better with that combination.

I would suggest you try finding, or casting your own flat-base bullets out of softer alloy, and see if you get better results.

BTW: To prevent gas cutting and for best accuracy, bullets need to be sized as close as possible to fit the throats, not the bore.

 

[Sunray]

Leading is caused by driving a cast bullet too fast. If you used the same data you used for your jacketed bullets, that'd be the problem.
Keyholing is caused by an oversized barrel or undersized bullets. Check the diameter of your cast bullets.
My GP(used it for years shooting Bullseye competition) shoots bevel based cast WC bullets with no problem. Hollow based WC's are much more accurate though.

 

[rcmodel]

Leading can also be caused by driving a hard cast bullet too slow.

Especially if the bullet is too small to fit the cylinder throats, and is too hard to bump up to fit them.

 

[BullitHolz]

OK some load info to bring this up to date.

Hornady bullets:
148 HBWC, 2.8 and 3.0 Bullseye, leads the throats, forcing cone and 1-2" into the lands.

158 LSWCHP 4.5 to 5.2 win 231, will keyhole the target with the first round. The same bullet over 3.1 Clays does the same thing.

I've used both .38 and .357 brass and there is no appreciable difference between the two types.

 

[rcmodel]

O.K.!

.38 Spl. or .357 cases?

I would buy a box of 158 grain lead SWC factory loads and try them.

If they key-hole on the first shot, your gun needs to go back to the mother-ship for repair.

If they don't, it's something wrong with your reloads.

 

[BullitHolz]

Both loads were tried in both the short and the long cases. I did try 3.2 of Bullseye in a .357 case under the HBWC. accuracy wasn't bad at all but they leaded the throats and the forcing cone up pretty good in 12 rounds.

 

[fecmech]

One other thing you might check is are you sure you have all the lead and jacket fouling out of the throats and forcing cone. If you push a tight patch from the muzzle end see if it starts to drag before it exits the barrel. If all the fouling is gone and there is no real tight spot where the barrel goes through the frame your only option is bigger bullets. Given your throat sizes I'd recommend .359 or .360 sized cast bullets, anything smaller with light .38 spl loads will lead badly.
BTW I have a Ruger .44 mag with a .429 groove and .433 & .434 throats. Shot fine with cast mag loads sized .430 and had minimal leading. Leaded badly with light .44 special loads due to gas cutting from blow by on the bullets. Cast and sized to .433 bullets and now have no leading at all with specials or mags. Size REALLY matters!

 

[straight-shooter]

I normally start by slugging the barrel. Then I cast my bullets and size 1 to 2 thousands over. If I get leading at the forcing cone and first 2 inches of the barrel that usually means the load is not hot enough. If the leading is half way up the barrel to the exit then it's too hot. Adjust the load accordingly and things get good from there.

My load for a S&W 686 6" barrel

38 brass
158gr swc sized to .357.5
unique 4.6gr
1.430 OAL
Barrel slugs out at .355.5

for the record my GP-100 also slugs at .355.5