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Posted: 8/18/2009 5:10:16 AM EDT
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Info:
Gun / Win model 70 bolt action 308 winchester Heavy Varmint 24 inch BBL Brass / WRA 68 Trimmed to 2.005 +/- .0002 Primer / Wichester large rifle Bullet / 168 gr nosler custom comp BTHP / Hornady 168 gr BTHP COL 2.800 - 2.808 Different loads Powder / Varget 42 grains Results 1 to 1.25 MOA Nosler. I had 1 group with 1/2 MOA but 1 straggler opened it up to 1 MOA. I dont know if it was me or the load. IMR 4895 / 39 grains Results 1/2 MOA Hornady Objective: nice light target load with accuracy to build on. Question: I have tried different loads and have found that the COL at 2.808 yeilded the best accuracy. Never went longer then that. The book says MAX COL 2.810. If I go to 2.810 will I produce a pressure spike? I made a dummy round with a 168 gr BTHP Hornady and let the bolt seat the bullet. I then pulled it out and the dummy round measured 2.912. I let the bolt pull it out. Is that my chamber length? The new load I want to try is 168 grain Nosler and Hornady BTHP with 40 grains of Varget and a COL of 2.810. Do you think it is safe? I believe the Max COL of 2.810 is tha SAMI number. Thanks in advance. |
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From the information you provided, it would appear your rifle likes the 4895 load. Depending on your intended use, I would go with .5 moa any day. Accurate enough for what is the question.
If you want to go further, seating longer might well give greater accuracy, but probably not by much is you are shooting .5 moa already. SAMI specs shold chamber in most any rifle. You can go longer in your gun, but you do need to know your chamber length. Pressure falls with longer COAL until the bullet hits the lands, then pressure can spike dangerously. Your method of measuring the chamber might not be accurate if the lands are holding the bullet and pulling it out of the case when you extract using the bolt. Either buy the Sinclair tool for this job, or load progressively longer and check for rifling marks on the bullet blackened with carbon. when you see rifling marks in the carbon, that's you chamber length. Sounds like you have a great start. Good luck. |
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Anon has it right.
I just wanted to add something about the COAL. This is the distance from bullet meplat to case head surface and can vary depending on the bullet design: FMJ, OTM, Polymer Tip, etc. Thus, the COAL can vary without changing the distance from the contact surface of the bullet ogive to the rifling lands when the cartridge assembly is seated in the chamber (bolt closed and locked, shoulder of the bottle neck case against the chamber). I am not familiar with the Winchester Mod. 70, but it may be that the magazine feed will not handle any cartridge assembly with a COAL greater than 2.810. In the Remington bolt I have, the magazine length is the limiting factor. The mystics of bench rest shooting like to hand feed, so they can eliminate magazine constraints. Then, they set the COAL for the particular bullet in play to position the ogive of the bullet at a certain distance from the rifling lands, up to and including interference fits. Finally, your tolerance on case trim length of +/- .0002 is a grinding tolerance and is typically measured in temperature controlled clean rooms. This infers some very precise brass, and could be partially responsible for your tight groups. |
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2.800" is SAAMI but for your/our purposes we can play with overall length. They are held to that length for sales to the general public which requires that the round will fit in any rifle.
Your brass is very old and has limited case capacity when compared to commercially manufactured brass. Be aware that 40.5 grains of IMR-4895 and a 168 match bullet is considered a maximum loading in military surplus brass. Sometimes age alone will cause brass to fail even though it has only been fired a few times, split necks are usually common with old brass. Hornady's latest manual has service rifle loadings in .308 listed seperate from their bolt action loads. When using this brass I'd stick with their service rifle listings and maybe lower. Do not exceed 2600 fps if you are looking for best accuracy. That is very close to a maximum loading with your brass. If you weigh an empty unprimed, resized and trimmed case it probably weighs close to 178 grains. Much heavier and thicker than standard brass. |
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