What’s the conclusion on 6.5 PRC twist rates? This gun will be used from sea level to 9500ft. Planning on 147eldm and hopefully heavier when the start coming out..l
PRC 7, 7.5 or 8 twist?
- Thread starter Gobears16
- Start date
Mine is an 8 twist and shooting mostly between 300-600 ASL. My shooting buddy has one too with same twist. Both are Kreiger 4-groove and we have both shot the 147 ELDM and new 153 A-Tip out to 1,000 yards with excellent results. I not that many shooters have experienced bullet blow-up with the 147 ELDM. I think most of them were shooting 7.5 twist so glad I went with the 8.
BTW, he shoots 147 with max load of H1000 and I shoot 140ELDM with a little lighter than max charge of Retumbo. We have found the rifles to shoot basically identical dope all the way down to that 1,000 yard target.
BTW, he shoots 147 with max load of H1000 and I shoot 140ELDM with a little lighter than max charge of Retumbo. We have found the rifles to shoot basically identical dope all the way down to that 1,000 yard target.
I have an eight twist Christensen MPR that gives me 1/4" @ 100 yards with the Hornady 140 gr. ELD-M, RL-26, and CCI BR-2 primers. This is the most accurate rifle I've ever owned and I've been at this for 50+ years. I've paid several thousand dollars for several customs over the years, all were fine, accurate rifles, but this MPR is my huckleberry! I don't think I could beat it with a stick.
If you have the 4DoF app, you can plug in all the variables you want to and create a table (bottom right button on the HUD) and it will tell you SG. I would personally run a 1:8" because I don't think that bullets are going to get much heavier/longer than they currently are. At most a guy could go 7.5", but a 1:7" is entirely unnecessary IMO. IME a SG over 1.2 is kosher and the BC loss isn't really that big a deal, but different strokes for different folks and all.. maybe you want to spin them faster.
FWIW, SG around 1.0-1.1 will create large groups and slightly oblong holes. SG over 1.2 will produce stellar accuracy, but may have some extra drag on the front end of the trajectory (read: lower BC). The RPMs drop slower than linear velocity, thus SG increases throughout the trajectory so SG at the muzzle is usually the most important thing to look at. There's no real benefit to SG's over 1.5-1.6. No real harm either, provided you're not spinning 300k rpm or faster.
ETA: One caveat... theoretically speaking, spinning bullets faster (understanding that every bullet has SOME amount of eccentric mass relative to the centerline of the form/shape) will create more dispersion.... However, this effect is largely a wash insomuch as barrel-to-barrel variation in precision capability is usually larger than the difference in 1-2" worth of twist and eccentric mass... If a guy were to test 50 barrels of each twist you might see a trend, though.
1:8 @ 2950fps-- 265500 RPM
147 ELD-M
Sea Level -20 deg F- SG= 1.41
Sea Level 59 deg F- SG=1.66
5000ft 90 deg F- SG=2.13
153 A-tip
Sea Level -20 deg F- SG= 1.24
Sea Level 59 deg F- SG= 1.45
5000ft 90 deg F- SG= 1.85
1:7.5 @ 2950fps-- 283200 RPM
147 ELD-M
Sea Level -20 deg F- SG= 1.61
Sea Level 59 deg F- SG= 1.89
5000ft 90 deg F- SG= 2.42
153 A-tip
Sea Level -20 deg F- SG= 1.41
Sea Level 59 deg F- SG= 1.65
5000ft 90 deg F- SG= 2.11
1:7 @ 2950-- 303428 RPM... Non-starter IMO. Asking for problems.
FWIW, SG around 1.0-1.1 will create large groups and slightly oblong holes. SG over 1.2 will produce stellar accuracy, but may have some extra drag on the front end of the trajectory (read: lower BC). The RPMs drop slower than linear velocity, thus SG increases throughout the trajectory so SG at the muzzle is usually the most important thing to look at. There's no real benefit to SG's over 1.5-1.6. No real harm either, provided you're not spinning 300k rpm or faster.
ETA: One caveat... theoretically speaking, spinning bullets faster (understanding that every bullet has SOME amount of eccentric mass relative to the centerline of the form/shape) will create more dispersion.... However, this effect is largely a wash insomuch as barrel-to-barrel variation in precision capability is usually larger than the difference in 1-2" worth of twist and eccentric mass... If a guy were to test 50 barrels of each twist you might see a trend, though.
1:8 @ 2950fps-- 265500 RPM
147 ELD-M
Sea Level -20 deg F- SG= 1.41
Sea Level 59 deg F- SG=1.66
5000ft 90 deg F- SG=2.13
153 A-tip
Sea Level -20 deg F- SG= 1.24
Sea Level 59 deg F- SG= 1.45
5000ft 90 deg F- SG= 1.85
1:7.5 @ 2950fps-- 283200 RPM
147 ELD-M
Sea Level -20 deg F- SG= 1.61
Sea Level 59 deg F- SG= 1.89
5000ft 90 deg F- SG= 2.42
153 A-tip
Sea Level -20 deg F- SG= 1.41
Sea Level 59 deg F- SG= 1.65
5000ft 90 deg F- SG= 2.11
1:7 @ 2950-- 303428 RPM... Non-starter IMO. Asking for problems.
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