FL members may recall I previously posted an accidental discovery of varying bullet base-to- ogive measurements in an old purchase of .270 Remington Cor-Lokt PSP 130gr bullets that I could never satisfactorily group in 4 different rifles.
I proceeded to measure 100 bullets from the base-to-ogive (BTO) and they ended up in 11 groups ranging from 0.588 to 0.574 inches. The difference suggested to me that if I seated the 0.588 bullet 0.02” from the rifling (leade), the longer 0.574 bullet would end up 0.014” further away, or 0.034” from the rifling if I seated it at the same die setting.
I then measured some Berger, Hornady and Sierra bullets of various calibers and found there were some variations, but not nearly as severe as those seen in the 130gr PSPs, which could very well be influenced by my measuring technique and precision of the Stoney Point comparator. But- there was even less variation when I measured match bullets.
I figured out how to compensate for the differences but other chores and duties interfered with my ability to load up some of those .270s and test the results.
But I ran into a situation last night which prompted me to share this with all of you.
I routinely load for my friend’s Sig 7mm Remington Magnum. He was shooting factory Federal 150gr Ballistic Tips and couldn’t get below a 1.5-inch group. I loaded Hornady 154gr SSTs, seated 0.02” from the rifling and the groups dropped to 0.716”. He has several 20-round boxes of the Nosler’s and decided he would just shoot them off. I suggested I would try to pull the bullets and reseat them to match the 0.02” seating of the SSTs to see if accuracy improved.
Here’s what I did:
BTO of 150gr Nosler BT = 0.724
BTO of 154gr Hornady SST= 0.717
The difference is the Nosler BTO is 0.007” longer.
The OAL, case base to ogive, of the Hornady 154gr at 0.02” = 2.750”
Subtracting 0.007” from that, the Nosler should be 0.02” from the rifling if seated at 2.743”.
Then, using the technique I learned from John Wooter’s “Practical Handloading,” I chambered the 2.750” SST (making certain the safety was on), inserted an aluminum cleaning rod into the muzzle and rested the tip on the bullet, then made a half-circle with a fine-tip black Magic Marker.
I chambered the 2.743” Nosler BT and made a full circle on the cleaning rod.
When I removed the rod, there was only one full circle, which indicates both bullets were seated 0.02” from the rifling.
Now my friend needs to fire the Noslers to see if accuracy improved.
This precipitated another thought. Suppose you shoot 130gr bullets in your .270 for deer and you want to load 150gr for elk. Your 130gr is accurate at 0.015” seating. Rather than using your usual process with a comparator to establish the cartridge base-to-ogive to the rifling for the 150gr, then seating from 0.01” and so forth, why not just measure the BTO of the 130gr and 150gr bullets and calculate as above to seat the 150gr at 0.015” and go from there?
I understand it’s not that simple, as bullets differ in shape, length, SD and BC, but I wonder if this process allows one to “jump the gun” to figure of what the bullet jump should be.
I proceeded to measure 100 bullets from the base-to-ogive (BTO) and they ended up in 11 groups ranging from 0.588 to 0.574 inches. The difference suggested to me that if I seated the 0.588 bullet 0.02” from the rifling (leade), the longer 0.574 bullet would end up 0.014” further away, or 0.034” from the rifling if I seated it at the same die setting.
I then measured some Berger, Hornady and Sierra bullets of various calibers and found there were some variations, but not nearly as severe as those seen in the 130gr PSPs, which could very well be influenced by my measuring technique and precision of the Stoney Point comparator. But- there was even less variation when I measured match bullets.
I figured out how to compensate for the differences but other chores and duties interfered with my ability to load up some of those .270s and test the results.
But I ran into a situation last night which prompted me to share this with all of you.
I routinely load for my friend’s Sig 7mm Remington Magnum. He was shooting factory Federal 150gr Ballistic Tips and couldn’t get below a 1.5-inch group. I loaded Hornady 154gr SSTs, seated 0.02” from the rifling and the groups dropped to 0.716”. He has several 20-round boxes of the Nosler’s and decided he would just shoot them off. I suggested I would try to pull the bullets and reseat them to match the 0.02” seating of the SSTs to see if accuracy improved.
Here’s what I did:
BTO of 150gr Nosler BT = 0.724
BTO of 154gr Hornady SST= 0.717
The difference is the Nosler BTO is 0.007” longer.
The OAL, case base to ogive, of the Hornady 154gr at 0.02” = 2.750”
Subtracting 0.007” from that, the Nosler should be 0.02” from the rifling if seated at 2.743”.
Then, using the technique I learned from John Wooter’s “Practical Handloading,” I chambered the 2.750” SST (making certain the safety was on), inserted an aluminum cleaning rod into the muzzle and rested the tip on the bullet, then made a half-circle with a fine-tip black Magic Marker.
I chambered the 2.743” Nosler BT and made a full circle on the cleaning rod.
When I removed the rod, there was only one full circle, which indicates both bullets were seated 0.02” from the rifling.
Now my friend needs to fire the Noslers to see if accuracy improved.
This precipitated another thought. Suppose you shoot 130gr bullets in your .270 for deer and you want to load 150gr for elk. Your 130gr is accurate at 0.015” seating. Rather than using your usual process with a comparator to establish the cartridge base-to-ogive to the rifling for the 150gr, then seating from 0.01” and so forth, why not just measure the BTO of the 130gr and 150gr bullets and calculate as above to seat the 150gr at 0.015” and go from there?
I understand it’s not that simple, as bullets differ in shape, length, SD and BC, but I wonder if this process allows one to “jump the gun” to figure of what the bullet jump should be.