Question on measuring slugs

[Sig]

Ok, I finally got around to slugging my P220 & GP100.

The P220 slug measured a heavy .453". This was carefully done using a cheap micrometer & 2 very good dial calipers. From what I've read(I'm new to casting), I should size my boolits to .454"-.455". Is there even a remote possibility that a RCBS .452" SWC mold that I bought will drop boolits large enough to be resized to .454"?

Now onto the GP100. How do you accurately measure a slug with 5 groove rifling?

Thank you for any insight into this.

[wallenba]

Yes, it's possible, depending on your alloy. Lyman number 2 usually does, and will rebound even more over time ('grow'). Most molds will measure wider across the parting line than 90* to it, so beagling tape may bring it where you want it also. Predicting how your mold of choice will perform is a **** shoot. I have identical molds that drop different.

Here's an RCBS SWC mold with three reviews. #2 indicates he can size .454 http://www.midwayusa.com/product/187705/rcbs-2-cavity-bullet-mold-45-201-swc-45-caliber-452-diameter-201-grain-semi-wadcutter

[Sig]

Thanks Wallenba,
I'll see if I can cast some this afternoon & see how they drop.

Any ideas on measuring the 5 groove rifling?

[wallenba]

Thanks Wallenba,
I'll see if I can cast some this afternoon & see how they drop.

Any ideas on measuring the 5 groove rifling?

Unless you have a very expensive 'V' anvil mike with the correct angle, I know of only one way. Buy a set of cheap feeler gauges. Measure the thinnest blade. Try to have one .001. Cut it free, then measure it with a micrometer to verify it's thickness. Wrap it tightly around the slug and measure with micrometer or good set of calipers. Then subtract twice the thickness of the gauge. Not perfect, but will suffice.

[Larry Gibson]

As with a 1 shot group measuring just 1 slug can be deceiving. Best to measure a minimum of 3 -5 slugs. Then if one of them is abnormal (for a variety of reasons) it can be discarded.

Larry Gibson

[runfiverun]

a sig at 453 seems waay out of spec.
maybe not waay out but about .002 out of where I would expect.

for the 5 groove, wrap something around the slug measure it
then subtract double the thickness of the wrap.
turn it 90* and do it again.

[Sig]

I will reslug the Sig barrel & check it again. I thought .453" was a little on the large side too. My method for slugging the barrel is to cast a blank in an empty piece of fired brass. Remove it with a bullet puller. Squash it a little so that it is .005" or so bigger than the bore & tap it though an oiled barrel with a brass rod.

I have an idea for measuring the 5 groove rifling. I have a set of small machinists "V" blocks. I was thinking of setting the slug in the "V" with the high spots touching the blocks & then taking a measurement from the base of the block to the top of the slug(also a high spot). Then replace the slug with different diameter drill bits till I get the same measurement. That drill bit should be the slug diameter. I'm probably over thinking this but I'm gonna try it.

[runfiverun]

it's not overthinking it if it makes sense to you.