Seating 75gr a max
- Thread starter Conor
- Start date
@Conor.
The book value C.O.A.L. for the 75gr A-MAX is 2.390".
Not 2.260". Where do you get that value from? Something sounds wrong buddy.
I assume your T3 Super Varmint is a 1:8" twist? I used to own that same rifle and shot the 75gr A-MAX. I needed an aftermarket magazine to deal with the loaded round overall length.
If you are pushing a 75gr A-MAX into the case to 2.260" with a warmish load, you might get a big surprise.
I'll try and find my notes on this load, few years ago now. IIRC the Tikka chamber won't allow for the 75gr to be seated out at book C.O.A.L. so you have to adjust powder weight down accordingly. As advised, get in touch with ChesterP.
The book value C.O.A.L. for the 75gr A-MAX is 2.390".
Not 2.260". Where do you get that value from? Something sounds wrong buddy.
I assume your T3 Super Varmint is a 1:8" twist? I used to own that same rifle and shot the 75gr A-MAX. I needed an aftermarket magazine to deal with the loaded round overall length.
If you are pushing a 75gr A-MAX into the case to 2.260" with a warmish load, you might get a big surprise.
I'll try and find my notes on this load, few years ago now. IIRC the Tikka chamber won't allow for the 75gr to be seated out at book C.O.A.L. so you have to adjust powder weight down accordingly. As advised, get in touch with ChesterP.
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I have a super varmint in 260rem. Crude DIY job with a junior hacksaw and the magazine fits 140grain eld match just nicely
Just bear in mind that the 75 gr Amax is quite a long bullet for this little calibre so make sure whatever distance from the lands you chose this round will still fit your magazine.
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ChesterP
Well-Known Member
Wow...PMax of 76.632 psi!
Must admit, I'd have chosen a slower burning /lower energy powder than N135 for the 75gr Amax...perhaps N140 if using Viht powders (I use RS50 for 77gr Sierra bullets).
However, the question above is what OAL to use. With that bullet, and the suggested max load it could be a heavily compressed load at 2.26OAL and as dodgy says, especially if close and personal with the lands, you could run into very high/dangerous pressure spikes. Stop and take a breath...
Using similar bullets, I started at 15 thou off lands albeit in a custom chambered rifle, with an oal of 2.352" (loads of 23gr to 24gr), and worked back to 2.29" for loads of 23gr or less in RS50. You'd be compressing loads a lot given your initial findings if working up to 24gr or anywhere close at 2.26OAL.
Personally, looking at the data, 24gr N135 for that bullet seated to 2.26 looks like it might be a heavily compressed load. If the OP suggests that rifling marks are evident on the dummy round, something's not right here and alarm bells should be ringing.
First question...how many times was the OAL checked for the dummy round? If using that (or any) method, I carefully re-check the oal at least 10 times and take the average. This method also shows if one or two checks which, for whatever reason, are a mile out. Check and double check, then check again 8 more times!
Once you've found your OAL, I wouldn't advise starting 10 thou off..it's too close to account for variations in meplat to ogive variations and you'll run into trouble, sooner or later. Really, you ought to check oal using a comparator gauge which measures to Ogive, not meplat. Start at 15 thou off, especially if using stiffish loads, no closer.
As you work your ladder up, one trick to ensure consistent uniform ignition and initial burn is to look at usable case capacity and seat the bullet to allow for 97% to say just over 100% case capacity. Slight compressed loads are no problem and on their own will not raise pressures. Pressures are more affected by being too close to the lands, especially where there can be significant variations in meplat to ogive distances and you've selected close seating based on meplat to case head measurement.
Slightly compressed loads won't crush the powder much....heavily compressed loads may. The trigger is that there is potential with the greater surface area and smaller particles for burn rate to be affected (speeded up) which can raise pressures. A light compressed load shouldn't result in this and will uniform burn rate for better shot to shot consistency, as with nearly case full loads.
If you start at 15 thou off and have a slightly compressed load for your maximum load that (say 101 to 103% useful case capacity fill ratio) then you should be on safe ground when working your charges up from the start load. That's what I'd advise anyway.
Something just doesn't look right though with your measured chamber length. The T3 in factory chambering in 1/8 twist (I'm assuming that's what it is?) ought to allow for bullets that take advantage of that twist rate unless you magazine length doesn't allow for recommended OAL in that bullet. If the rifle is set up for longer bullets from the factory, then my first common sense check would be to re-test OAL checking very carefully that everything is set up correctly and repeating for an average as above. That has to be the starting point before starting on any load development.
Must admit, I'd have chosen a slower burning /lower energy powder than N135 for the 75gr Amax...perhaps N140 if using Viht powders (I use RS50 for 77gr Sierra bullets).
However, the question above is what OAL to use. With that bullet, and the suggested max load it could be a heavily compressed load at 2.26OAL and as dodgy says, especially if close and personal with the lands, you could run into very high/dangerous pressure spikes. Stop and take a breath...
Using similar bullets, I started at 15 thou off lands albeit in a custom chambered rifle, with an oal of 2.352" (loads of 23gr to 24gr), and worked back to 2.29" for loads of 23gr or less in RS50. You'd be compressing loads a lot given your initial findings if working up to 24gr or anywhere close at 2.26OAL.
Personally, looking at the data, 24gr N135 for that bullet seated to 2.26 looks like it might be a heavily compressed load. If the OP suggests that rifling marks are evident on the dummy round, something's not right here and alarm bells should be ringing.
First question...how many times was the OAL checked for the dummy round? If using that (or any) method, I carefully re-check the oal at least 10 times and take the average. This method also shows if one or two checks which, for whatever reason, are a mile out. Check and double check, then check again 8 more times!
Once you've found your OAL, I wouldn't advise starting 10 thou off..it's too close to account for variations in meplat to ogive variations and you'll run into trouble, sooner or later. Really, you ought to check oal using a comparator gauge which measures to Ogive, not meplat. Start at 15 thou off, especially if using stiffish loads, no closer.
As you work your ladder up, one trick to ensure consistent uniform ignition and initial burn is to look at usable case capacity and seat the bullet to allow for 97% to say just over 100% case capacity. Slight compressed loads are no problem and on their own will not raise pressures. Pressures are more affected by being too close to the lands, especially where there can be significant variations in meplat to ogive distances and you've selected close seating based on meplat to case head measurement.
Slightly compressed loads won't crush the powder much....heavily compressed loads may. The trigger is that there is potential with the greater surface area and smaller particles for burn rate to be affected (speeded up) which can raise pressures. A light compressed load shouldn't result in this and will uniform burn rate for better shot to shot consistency, as with nearly case full loads.
If you start at 15 thou off and have a slightly compressed load for your maximum load that (say 101 to 103% useful case capacity fill ratio) then you should be on safe ground when working your charges up from the start load. That's what I'd advise anyway.
Something just doesn't look right though with your measured chamber length. The T3 in factory chambering in 1/8 twist (I'm assuming that's what it is?) ought to allow for bullets that take advantage of that twist rate unless you magazine length doesn't allow for recommended OAL in that bullet. If the rifle is set up for longer bullets from the factory, then my first common sense check would be to re-test OAL checking very carefully that everything is set up correctly and repeating for an average as above. That has to be the starting point before starting on any load development.
Stability in my 8” twist 223 Has been marginal depending on temperature but as it’s only a 20” barrel, I’m a little down on velocity from optimum so hey-ho, such is life if I’m chasing the lands, i never rely on books, I measure the OAL in my own rifle and work from there in 5 thou steps
ChesterP
Well-Known Member
I've tried 77gr loads (TMKs) and they stabilised just fine in my 1/8, but if the Amax uses longer bullet sidewall length then I guess stability could start to get marginal. A 1/8 should be able to stabilise 80grn smks and I know of plenty who shoot those very well.
Looking at Brian Litz's data on stability, the following stability factors apply to the Amax:
Barrel twist: 1/8
Best case: 1.42
Nominal: 1.24
Worst case: 1.07
Based on that and that alone, it would appear that 75 Amax are not best suited to a 1/8 twist, instead requiring 1/7" twist or more to stabilise.
I'd be looking more towards the 52grn Amax or the 77grn Sierra TMK as a better match for that barrel. I have shot the latter into very tight groups from my own 1/8 .223
Looking at Brian Litz's data on stability, the following stability factors apply to the Amax:
Barrel twist: 1/8
Best case: 1.42
Nominal: 1.24
Worst case: 1.07
Based on that and that alone, it would appear that 75 Amax are not best suited to a 1/8 twist, instead requiring 1/7" twist or more to stabilise.
I'd be looking more towards the 52grn Amax or the 77grn Sierra TMK as a better match for that barrel. I have shot the latter into very tight groups from my own 1/8 .223
ChesterP
Well-Known Member
Back to the OP...there seems to be two main issues: the Viht published OAL for the 75 Amax V's their max load...that just doesn't look right to me at all! Secondly, the measurement check to do on your own AOL. Start from there and work back and you wont go wrong, but as above, I think you might struggle to stabilise those...
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