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Old March 9, 2015, 12:04 PM   #1
Join Date: February 14, 2014
Posts: 40
Help me understand these #'s! DOPE vs chrony vs QL...

I need a little advice working through this stuff. I just got back from a range session with a new(ish) rifle. It's a Savage based homebrew frankenrifle in 260 Rem with a 24" CBI barrel. At the end of last season I had worked up an accuracy load with 41.5gr of H4350 behind the Berger 140gr hunting VLD (#26504, older version with 0.595 G1 BC). The scope is a Sightron SIII 6-24x50LRMOAFFP.

Anyway, this load goes over my ProChrono digital at an average of 2775fps. If I stick that in my Android ballistics computer (Strelok+) with a 100yd zero it spits out a vertical correction of 12.92 MOA at 600yds.

I recently bought a copy of Quickload and when I plug in all the data and dimensions (accounting for measured case capacity, trim length and COAL) for the load it spits out 2674fps right at 54K psi which is darn close to what the Hodgdon data says it should be from a 24" barrel. It's very easy for me to believe that the cheapy chrono could be 100fps fast, BUT...

Today I took that soft DOPE and stretched it out to a real-world 600yds on our high-power range. The actual correction turned out to be 12.25 MOA.

Going back into Strelok and using the trajectory validation feature, 12.25 MOA equates to a velocity of 2841fps! So--2674 from QL, 2775 from the chrony, and 2841 from the DOPE.

What bothers me is that if I go back into QL and mess with the case capacity to get a velocity of 2841fps from 41.5gr of H4350, it turns out a pressure just over 72K psi. QL wants that adjusted load to be compressed 112%. The actual load is not compressed at all. Personally, I don't think 72K psi is imminently dangerous in this rifle, but it is quite a bit higher than I expected. If it really is up there then I'll probably try and find a lower velocity node to work with.

The question is, what's actually going on in my chamber? To me the 600yd DOPE is hard to argue with. It is what it is. So why are my other numbers so far off?

I have not yet had a chance to check to scope tracking at 600yds, but a box test at 100yds is dead-nuts on.

Last edited by obiwannabe; March 9, 2015 at 12:16 PM.
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