MP-359-125 help

[Charlie Two Tracks]

I have a MP 359-125 mold that I am trying to use for .38 rounds. The BHN is 8.2 and I have beagled the mold to drop at .360 to .361. I need this diameter for my Ruger SP-101 that I reamed the cylinders on. Anyway. When I try to load a round and then pull it to check the diameter, it is really reduced. I started out with a .3614 boolit and when pulled it is .358. I am using a Lyman M die and I am flaring the case so much that it is hard to enter the RCBS seating die. I am using very little crimp. The rounds push tight into the cylinder. Is the lead too soft for this type of mold? The driving bands are very small. I would like to keep it at .359 when finished. I am trying to get a good expansion of the HP on this boolit.

[MikeS]

Have you checked your seating die to see if it's swaging down the boolit? If you're using 38 Special brass, and flaring it with an M die, I can't think of anything else that might be making your seated boolit go down to .358. Another thing, what size M die are you using? I believe there are 2 different 38 M dies, a 38P and a 38AP, your want the 38P as the 38AP is really for 9mm and might not be expanding your brass enough. Remember the M die is a 2 step die, and if the smaller step isn't expanding enough, that could cause your boolits to get swaged down to .358 too.

[Charlie Two Tracks]

I just took my M die apart and measured the diameter with my mic. Right against the shoulder that bells the case (2nd stage) it measures .3557? Should it be like that? Seems small to me. The die has 38P stamped on the bottom of it.

[cbrick]

Don't need more flare, just enough to start the bullet without touching the edge of the case mouth. Any more and all your doing is working the brass more than needed.

The Lyman plug at .3557" would probably be fine except you have a rather large bullet. You said they chamber hard so you could use a smaller sizer die, if your throats are reamed at .360" try a .360"die remembering that many alloys will size slightly smaller than marked die diameter but even a .359" die could possibly work. Chambering hard is not what is needed, a mild snug fit is good. What does this mold drop bullets at without beagling it, you may well not need to. If they are close to .359" without beagling try to fit a couple through the cylinder throats without any sizing to see how they fit, if even close use a larger sizer die to lube only, no sizing.

NO, your alloy is not too soft. Doesn't matter what the BHN is it is not too soft for a 38 Special.

Rick

[462]

I had similar problems with a .357 Magnum Blackhawk and a pair of S&W .44 Special's.

1. After determining that the Lee seating dies were swaging the boolits to a smaller diameter, I switched to Lyman dies and eliminated the problem. (My experience has been that Lyman seating dies are machined to accept slightly fat-for-calibre cast boolits, whereas other brands aren't, though I haven't tried RCBS's cowboy dies.)

2. Since M-die expander plugs are dimensioned for jacket bullets, I had larger expander plugs made up. It may not work for anyone else, but for both plugs, I had the first step made .002" smaller than the boolits' base and the second step .001" larger.

[leftiye]

Yuu fer sure could use a bigger expander ball. .358 or .359 would be correct for .360 to .361 boolits. This could fer sure be sizing your boolits down when you seat them, especially if your lead is on the soft side.

Since your chamber mouths have already been enlarged, you may have to thin your case necks or hone out your chamber neck area a thousandth to stop the hard chambering. This (tight fit) will raise pressures.

Before you do anything like this though make sure what part of the cartridges are interfering with what part of the chambers. Cover the cartridge with magic marker black, and chamber them to see where the black rubs off.

[Charlie Two Tracks]

I agree with the bigger expander but I don't know how you do that. I'm going to quit sizing my brass for the .38 and see if that helps. The Lyman epander has all kinds of room in a fired, non sized case. All the play in the world, so it really does nothing to the case. It's when I size it that it fits tight. Come to think of it, I really don't know why I am sizing the brass back to original form......... we will see.
The reason for the tight fit in the chambers is that I bell the end of the brass so much and then just barely have the crimp die bring it back to the boolit.