Re: Maximum COAL to fit in TRG 42 338 Lapua mag?
I received a PM that tells me that others are working on the same objectives. The PMer asked some questions which I will try to answer in this posting:
Here is what I have done so far:
Because loading "into the ands (i.e. negative jump) is apparently not possible on TRG 42 338 rifles because of magazine length limitations, I have targeted loading for .010" short of the lands, or shorter. i.e. .010" is the closest I want to get to the lands.
I measured a sampling of the Sierra 300g Matcking 338 bullets to determine both bullet-base-to-ogive (BBTO) dimension and bullet overall length (BOL). I found a substantial variance on these individual measurements, from smallest to largest. I found that BOL cares by .011" from longest to shortest, and BBTO varies by .008" from longest to shortest. I calculated the Ogive-to-Tip (OTP) dimension for each bullet in my sample.
I picked the bullet with a representative BBTO (just .0005" longer than statistical average) but with the LONGEST OTP dimension, reasoning that this would be the bullet that would generate the longest COAL that could potentially make the round too long for the magazine. I MARKED that bullet and used it as my sample bullet with the Stony Point (Hornady) OAL gage. That gage plus my digital calipers told me that my "worst case bullet" hit the lands at COAL = 3.7425. Note that I measured COAL, not case-base-to-ogive. This is for two reasons:
1. The limiting factor is the magazine, which I am led to believe (elsewhere than the Hide) will accept up to a 3.73 or 3.74" COAL. (Bear with me, see below)
2. It is impossible to accurately measure case-base-to-ogive (CBTO) with the Stony Point gage holding the round, using bullet ogive insert and my digital calipers, as with this Sierra 300g Mkg bullet the whole assembly cannot be made "straight" enough to prevent cocking error at the bullet to ogive insert interface due to the Stony Point gage construction (try it and you'll see what I mean). Measuring the COAL is much more accurate.
Once I had the 3.7425 maximum COAL, I was able to determine CBTO accurately because I KNOW the ogive-to-tip measurement, so simply subtracted that from COAL to get CBTO. THIS (CBTO) is what I measure in "production", since CBTO varies FAR less than COAL, because the bullet manufacturer holds BBTO much closer than bullet OAL.
So, wanting to start at .010" off the lands for my load testing, I target:
COAL = 3.7425 minus .010" = 3.7325".
To determine the CBTO, I subtract ogive-to-tip for my sample bullet (.8605") from COAL of 3.7325, and get 2.8720". BUT, my specific sample bullet's ogive is .0005" higher from bullet base than the AVERAGE of the bullets. So, I subtract .0005" and get:
target CBTO = 2.8715".
This CBTO must be my MAXIMUM in production (so that I stay safely .010" off the lands, and I know that my bullets at worst will vary up to .008" on CBTO (.011" on COAL!) due to Sierra's tolerances, so my production range is:
CBTO (case-base-to-ogive) = 2.8715" +0" / -.008"
and COAL from above = 3.7325" +0" / -.011"
Now, I may find that a COAL of 3.7325" might be too long for my magazine. I'l find out by trying to load 5 rounds into the magazine and seeing if there are any loading or feed hiccups. If so, I'll simply shorten each cartage in the prototype production batch by running them through the seating die again, with a shorter die setting, until they DO load and feed reliably.
Hopefully all the above makes sense to a patient reader.
Jim G
