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Have a first series Post 64 M70 in 7mmMAG, with commercial ammo I have groups that might group on a paper plate, @100 yds., even local shop couldn’t get a decent group.
My reloads with Hornady 162gr, SST, are loaded to meet the lead / throat, @ 2.792, using a “Sinclair comparator [hex nut].
SAAMI chamber drawing indicates the basic dimension from bolt face to “leade / throat” is 2.725″. [difference = 0.067″]
Just keep loading to the leade or look for another barrel??
My handloads show a group of 1″ @ 50 yd for chronographing.
Also, 4 or 5 reload brass aredeveloping shoulder crack from firing, is the chamber not made to dimensions??
Thanks.
JHR
A few thoughts or question:
- Has the action been stretched from too many high pressure loads? More specifically, is the distance between the bolt face and the breech face within spec?
- That is a pretty big difference (0.067″). You don’t mention the tolerance but that is almost 1/10″.
- Questions and worries about (1) and (2) aside, I would fire-form the brass and then load to the leade as you are doing, but I would not resize the cases once they are fire formed. As you have found out, your hand loading to the leade results in an improvement in accuracy but resizing will result in cracked cases where the shift in dimensions is greatest during fire forming. If the bullet is not a reasonably tight fit in the case, then I would neck size only. Working the brass back and forth between the dimensions of your sizer and the dimensions of your chamber will greatly reduce brass case life. Annealing can help recover the elasticity of your cases for those that have not cracked yet.
JHR
Generally when trying to track down accuracy issues, I start with the simple stuff first:
Are you shooting with a scope? If you are, check that the scope is well mounted and not moving under the recoil.
Check the rifle’s bedding. I recently purchased a post 64 MOD 70, built in the early 80s. This rifle would not hit a paper plate at 50 YDS much less at 100. Drove me nuts, finally pulled the action from the stock, and to my surprise found a piece of index card as shimming to keep the action from rocking in the stock; reading what was written on the index card, this was factory work.
Looking at rifles built in the 90s, looks like they use a bead of hot melt glue to bed the action, so, not a whole lot better.
After properly bedding the action, that rifle now shoots well.
On the split cases, couple of questions:
Does this happen only with reloads, or commercially loaded ammo as well?
If only with reloads
Are you using virgin brass?
If not, how many times have those cases been reloaded? Are you keeping track.
As 38-55 noted, annealing of the cases may be required, the brass tends to work harden, annealing softens the brass for a little more useful life.
If you’re concerned about the chamber, before switching barrels, have someone do a chamber cast to verify that the chamber is correct.
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