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+--- Thread: McGowen Precision AR-15 6-ARC build (/showthread.php?tid=7)
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RE: McGowen Precision AR-15 6-ARC build - grayfox - 08-31-2020
I should make a couple other observations. True bearing surfaces for 2 Sierra mk’s 6mm and 6.5mm would differ as you say (probably) but you really need to look at the 2 surfaces, after all the rear BT lengths could differ... but in general I would agree. That correlation weakens more, however, when you take 2 from different mfrs. but still can be a factor. 2d, I don’t look for max mv’s from either, I’m looking for mv’s within the same node range for 20” barrel... that’s why I picked out 2700. Trashy’s 20 in the 6arc shoots much faster than even mine and maxes definitely above 2700, but that’s all I need for my purposes. So that’s why I picked out the 2700 figure... max mv’s isn’t my goal.
RE: McGowen Precision AR-15 6-ARC build - StoneHendge - 08-31-2020
I was joking about the chrony - I haven't figured out where the smileys are on this forum yet.
I would hope they're using radar!Hornady's data says they are using a Noveske barrel. My Proof 6.5 CM barrel runs a tad slower than what most data predicts but it's not significant - maybe 20-40 fps. I do take the time to find the powder that's going to give me the best node (SD and accuracy) approaching max velocity.
Putting both 105 grain Nosler RDF's and 105 grain Berger Hybrids next to Hornady 105 HPTB on the screen and they are definately sleeker and certainly appear to have less bearing surface so a little can be explained there. In fact, it's such a pig that the Berger twist calculator says my 1:8 can stabilize it while the RDFs and Berger Hybrids get compromised.
But even if I look at my LBC vs Hornady and account for smaller bearing surface of 95 SMK vs 95 SST, slightly larger case, BHW-ness of barrel and maybe being slightly over 52k PSI, I'm still getting 2750 vs the 2600 Hornady gets using the powders that usually give top velocity (at the cost of temp sensitivity)
RE: McGowen Precision AR-15 6-ARC build - Trashy - 08-31-2020
What is the case capacity difference between the LBC and the ARC? Would that explain any of the difference?
RE: McGowen Precision AR-15 6-ARC build - Pig_Popper - 08-31-2020
Trashy - before 6-ARC I’d of said yes but If you look at 6mm PPC and 6-ARC water case capacity difference it has no change in velocity (based on Hornady’s anemic data).
Stonehenge you missed my sarcasm on the 200 FPS comment - that’s 200 FPS faster than Hornady’s load data not necessarily actual MV in real life.
One benefit for the wildcatting calibers is that the one step neck down from 6.5 Grendel brass can use sturdier cases like Norma and Lapua - I.e. a little hotter loads can be run using the die setups and these brass types.
If you look at 6-ARC on the surface you don’t have that sturdier brass available yet and I don’t believe you can just downsize 6.5 Grendel brass in one motion with the 6-ARC dies although I saw a YouTube video this past week of a guy showing to do just that ...
I like all the speculation gents - it certainly is interesting how the Hornady marketing team has gone quiet the last month now that this data is released - it’s sort of hard to explain I suppose even for the “creators” of the caliber.
I’ll tell you this : if I were shopping for 6-ARC and had no previous knowledge of 6.5 Grendel and other 6mm wildcats and I saw the load data MVs I would pass - meaning Hornady would lose a possible customer and this forum would lose a possible new member.
The correct data IS that important !
RE: McGowen Precision AR-15 6-ARC build - StoneHendge - 08-31-2020
Trashy, all of the water capacity measurements I see in the world wide web are all over the place. The 6 ARC measures 0.9207" from the top of the case head to the bottom of the shoulder while the 6.5 Grendel (and hence 243 LBC and 6 Grendel) measure 0.9507", so about 3.2% (or actually a little less since an equal amount of space is added for the area around the bullet in the shoulder).
RE: McGowen Precision AR-15 6-ARC build - grayfox - 08-31-2020
picture. (Minor edit: the 2 trendline formulas on the top 2 curves are actually reversed, the 0.99 one is for trashy's.)
RE: McGowen Precision AR-15 6-ARC build - Trashy - 08-31-2020
Grayfox,
Is the graph showing the data points between our barrels and hornady’s published data using LVR & the 95 SST’s?
RE: McGowen Precision AR-15 6-ARC build - grayfox - 09-01-2020
Yes, everything is the 95 sst and lever powder.
Yours is the blue line, mine is orange. Hornady is on the bottom. Slowest speeds, now a piece of that (50 ft/s or so) is due to it being an 18” barrel. But still quite a delta.
Yours is about 240-250 faster, mine is about 180.
RE: McGowen Precision AR-15 6-ARC build - Trashy - 09-01-2020
Grayfox,
Thank you sir for the follow up! We both had different velocity goals which were reached so the differences are understandable and I think proportionally related. Thanks for the graph.