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 Post subject: Base to Ogive measurement 6.5 CM/Tikka CTR
PostPosted: Thu Feb 21, 2019 1:49 pm 
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Joined: Wed Feb 06, 2013 10:31 pm
Posts: 221
I purchased the Hornady OCL gauge and comporator set recently.

I measured the Base to Ogive (BTO) of three bullets I have been using in my Tikka T3x CTR chambered in 6.5 CM. The Tikka has a long throat and a magazine to match (2.915 OCL possible).

I have a lot to learn about technique and some of my variation is no doubt lack of familiarity w the tools, but I’ve spent 4-5 hours w them and am getting more consistent as I learn.

I have been shooting the 140 Nosler RDF, 140 ELD and 147 ELD w RL-16 and Hornady brass. Federal 210-M primer.

43.6 grains works well w the 140’s and 42.8 w the 147. 2810-2825 fps for the 140 and 2725 fps for the 147.

The 140 Hornady ELD is the best match for BTO and OCL. It fits in magazine at both 20 and 30/1000 off jam. 140 RDF is GTG at 60/1000 off jam (which it prefers to closer spacing). The 147 barely fits at 45/1000 and may be best left for the magnum calibers.

The 140 ELD BTO was most consistent to measure. 1/1000 fairly typical variation. Longer 147 ELD and 140 RDF more like 4-5/1000 and once even 9/1000. Since they are longer, they may tip more and give me more erroneous readings? (Or it might just have been a primer not seated perfectly).

The 140 RDF is worth putting some effort into as it has the real world BC of a 147 w the velocity of a 140. (Even a bit faster than the 140 ELD). But the 140 ELD is just plain easy to load for.
Here are my results.

140 RDF 2.3175 to jam, minus 60/1000 = 2.2575, or about 2.910 OCL. (It is a factory pointed bullet and has very uniform OCL). This fits in Tikka magazine but near maximum.

140 ELD 2.2835 to jam, minus 20/1000 = 2.2635 or about 2.901 OCL. Best fit.

147 ELD 2.3185 to jam, minus 30/1000 = 2.2885 or about 2.926 OCL, single feed only. 45/1000 jump to fit maximum magazine length at 2.9105. If I had a heavier rifle I would probably use RL-26 with this bullet, but it has a very high flame temperature and did not have as many “flat” points in a ladder test as the 140’s.

All of the bullets are accurate even w deeper seating depth but 140 ELD is by far easiest to fine tune.

Comparing BTO to OCL shows the 140 RDF at 4/1000 variation, 140 ELD at 5/10000 to 4/1000 variation, and 147 ELD at 15/10000.

Since I am shooting a 9.5 pound rifle and have 60 plus year old eye balls, I do not have an accurate enough setup to justify time spent measuring BTO and OCL. But I did learn to seat 140 RDF and 147 ELD as far out as possible, and to try the 140 ELD at both 20/1000 and 30/1000 jump.

The comporator was very useful in helping me adjust my resizing die. I was over sizing previously.

If any other Tikka shooters out there have measurements I’d love to hear from you, as like I said, I am still learning how to use the tools consistently.

What I have learned about the OCL gauge is to make sure the modified cartridge is screwed on tight before and after measuring, make sure the caliper lays flat against the flat of the gauge, and make sure the set screw is snug on both the insert and calipers.

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