Interesting numbers loading 7mm mag
Posted: 30 Dec 2017 14:45
I know pretty much all of us (?) hate math but I ran into an interesting issue while loading for my "new to me" 7 mag.
Components:
RP once fired brass (not from my rifle) full length sized with Forster BR sizing die. Did not "bump" shoulder as the brass cycled fine in my chamber. Brass was trimmed to minimum of 2.490 per Speer reloading book. All were within 2 thousandths.
Bullets are Speer 145gr. SPBT (hence the Speer manual) load data is unimportant as this is about OAL, distance to lands etc; (however the load data could be affected by differences in the seating depth)
Manual says OAL as tested is 3.280 using this bullet.
So I proceed to load up 12 rounds using this number (I know base to tip measurements can vary a bit because of the lead tip irregularities but should still be within a couple thousandths) using Forster BR dies and a co-ax press. (means nothing in the overall scheme of things) just like most would do (I believe).
Well guess what? Rounds wouldn't chamber in my rifle....
So out comes the Hornady "head space" gauge (really a head space comparator) set up to find distance to the lands from the ogive of this bullet. Measurement comes out at 2.730 (this number means nothing by itself except as it applies to the other measurements taken with the bullet and case comparator) The "head space gauge" uses a modified case made by Hornady for measuring so you need to compare it to the cases you're using (shoulder location) The modified case is 2 thousandths shorter in this area than the cases I'm using so that has to be taken into account when figuring seating depth. I decided to seat 20 thousandths off the lands (just a place to start) so subtract 20 thou from the 2.730 and we have 2.710 PLUS the two thousandth difference between the modified case and the once fired case, so we're now at 2.708. This should put us 20 thou of the lands OF MY RIFLE.......told ya there would be math involved....
Bullets were seated using the above mentioned Forster BR seating die and all came out within a thousandth of each other measured with the Hornady bullet comparator. Here's where it got interesting to me (really all of this was/is "interesting") when I measured the OAL of my finished rounds with a good set of calipers (aerospace) they come out at 3.198... .082 thousandths SHORTER than the book shows as tested. Even if you load to the lands on this rifle you'd still be somewhere around 60 thousandths shorter than their number. Wonder what kind of chamber they were using as a test platform? I did measure some factory Federal 150gr SP to compare and these were 20 thousandths SHORTER than my reloads. I've come to the conclusion that some method of making these measurements is more important than what I USED to think. I know I'll not reload another bottleneck rifle round without first checking this. Does anyone see anything I missed or did incorrectly?
Components:
RP once fired brass (not from my rifle) full length sized with Forster BR sizing die. Did not "bump" shoulder as the brass cycled fine in my chamber. Brass was trimmed to minimum of 2.490 per Speer reloading book. All were within 2 thousandths.
Bullets are Speer 145gr. SPBT (hence the Speer manual) load data is unimportant as this is about OAL, distance to lands etc; (however the load data could be affected by differences in the seating depth)
Manual says OAL as tested is 3.280 using this bullet.
So I proceed to load up 12 rounds using this number (I know base to tip measurements can vary a bit because of the lead tip irregularities but should still be within a couple thousandths) using Forster BR dies and a co-ax press. (means nothing in the overall scheme of things) just like most would do (I believe).
Well guess what? Rounds wouldn't chamber in my rifle....
So out comes the Hornady "head space" gauge (really a head space comparator) set up to find distance to the lands from the ogive of this bullet. Measurement comes out at 2.730 (this number means nothing by itself except as it applies to the other measurements taken with the bullet and case comparator) The "head space gauge" uses a modified case made by Hornady for measuring so you need to compare it to the cases you're using (shoulder location) The modified case is 2 thousandths shorter in this area than the cases I'm using so that has to be taken into account when figuring seating depth. I decided to seat 20 thousandths off the lands (just a place to start) so subtract 20 thou from the 2.730 and we have 2.710 PLUS the two thousandth difference between the modified case and the once fired case, so we're now at 2.708. This should put us 20 thou of the lands OF MY RIFLE.......told ya there would be math involved....
Bullets were seated using the above mentioned Forster BR seating die and all came out within a thousandth of each other measured with the Hornady bullet comparator. Here's where it got interesting to me (really all of this was/is "interesting") when I measured the OAL of my finished rounds with a good set of calipers (aerospace) they come out at 3.198... .082 thousandths SHORTER than the book shows as tested. Even if you load to the lands on this rifle you'd still be somewhere around 60 thousandths shorter than their number. Wonder what kind of chamber they were using as a test platform? I did measure some factory Federal 150gr SP to compare and these were 20 thousandths SHORTER than my reloads. I've come to the conclusion that some method of making these measurements is more important than what I USED to think. I know I'll not reload another bottleneck rifle round without first checking this. Does anyone see anything I missed or did incorrectly?