What if...for slimjim by request, or anyone else, feel free.

slimjim

fnuser wrote:

but I still like the 6.8 over the 6.5 for the ability to use standard mags with a slight mod not possible w the 6.5

I thought that 6.8 SPC required a different mag than the 5.56/.223. Do you have some information showing how to use a standard 5.56/.223 mag?

fnuser

yes... but I am hesitant to release this data before I have confirmed it personally. Ya know not perpetuating a myth.

slimjim

fnuser wrote:

Ya know not perpetuating a myth.

I agree with not pepetuating mis-information.

slimjim

Since learning about the 6mmBR, I've been investigating what caliber options are available based on that case. I most likely would fireform to BRX with either the 6mm or 6.5mm (moves shoulder .1 inches further forward). Here is a similar analysis. I had to make some initial velocity assumptions with a 24" barrel so will list the data I used for the graph in the next post so shooters can comment. I tried to use information listed on other forums and the assumption that the BRX case would yield an extra 150 fps with the extra powder capacity.

slimjim

data for the above graph

6mm BRX _____________ BC _ velocity _ 1K yds
65 gr _ Hornady _ V-Max _ 0.28 _ 3500 _ 1016
85 gr _ Barnes _ TTSX ___ 0.33 _ 3224 _ 1095
95 gr _ Sierra _ MK HP ___ 0.45 _ 3127 _ 1390
107gr _ Sierra _ MK HP __ 0.52 _ 3012 _ 1495
83 gr _ GSC ____ SP _____ 0.50 _ 3225_ 1580

6.5 Grendel
90 gr _ Speer _ TNT HP __ 0.28 _ 2850 __ 905
108gr _ Lapua _ Scenar __ 0.47 _ 2700 _ 1230
123gr _ Lapua _ Scenar __ 0.54 _ 2600 _ 1301
130gr _ Berger _ VLD ___ 0.55 _ 2492 _ 1248
140gr _ Berger _ VLD ___ 0.61 _ 2220 _ 1187

6.5mm BRX
90 gr _ Speer _ TNT HP _ 0.281 _ 3350 _ 989
100gr _ Barnes _ TTSX __ 0.41 _ 3200 _ 1314
108gr _ Lapua _ Scenar __ 0.48 _ 3100 _ 1448
123gr _ Lapua _ Scenar __ 0.55 _ 2900 _ 1481
140gr _ Berger _ VLD ___ 0.61 _ 2550 _ 1376
110gr _ GSC _ SP ______ 0.53 _ 3100 _ 1573
120gr _ GSC _ SP ______ 0.62 _ 2850 _ 1573

fnuser

i am not seeing the energy #'s on your graphs i see them in the second set of data but am not able to extrapolate a quantity based on the graph alone. However the muzzle energy per g of powder compares favorably between the 6.8 and 6.5 brx

slimjim

I did not display muzzle energy in the graph, just ME/grain of powder. When you go to the next heavier pullet and you get the ME/gr pwdr dropping off, I use that as an indication that you might be going past the optimum bullet-cartridge capability.

I also need to do some more checking on these velocities. Most BRX shooters are getting these velocities with 28" to 31" barrels. I've seen a couple get these belocities with 24-inch barrels so I need to understand what what the differences are.

slimjim

fnuser wrote:

what about that other upper you showed me that the side handle was independent of the carrier?

I've decided that I have no issue with a recipocating side charge handle. I have one on my M1 and Mini-14 and there never was an issue. I'm almost ready to buy the upper receiver but have too much going on to work and want to make sure I have a clear mind when I start spending the money.

fnuser

using the numbers from your graph the 100g 6.8 bullet is getting @1740 ftlbs. this beats all the contenders in the second graph. Remington lists their factory 115 at 2000 ftlbs I know this sounds kinda measly but its in the same ballpark as factory 257 Roberts and similar cartridges. and if we choose to limit aourselves to the ar-15 frame this is just about what we can expect, the ar-10 frame would be another story completely. Reports from the field are good for performance on game including hogs. This is as about as far as I would push it anyway. I think if you are just punching paper 6mm norma br would be as good as it gets. and would be pretty easy to come up with components, the brx might be as nice but with the added case forming and obtaining dies in a timely manner, I don't know if it would be worth it unless I got bit pretty hard by that bug. I'm still scratching where the .257 Ugalde got a hold of me. Maybe a cute little Remington #7 or Mini Mauser is in the future for that.

fnuser

You are right in saying the good old .270 Win is hard to beat that's what made it so good and old.

slimjim

fnuser wrote:

the 100g 6.8 bullet is getting @1740 ftlbs. this beats all the contenders in the second graph.

I added ME below. The BRX case has more capacity than the 6.8 SPC. If you increased the bore of the BRX to 6.8, I'm sure it would have greater ME than the 6.5. The 6.8 would be superior for short barrels and short range but the lower BC would reduce energy faster and result in less power at mid- and long-range.

6.5mm BRX __________________ MV ___ ME
90 gr _ Speer _ TNT HP _ 0.281 _ 3350 _ 2243
100gr _ Barnes _ TTSX __ 0.41 _ 3200 _ 2274
108gr _ Lapua _ Scenar __ 0.48 _ 3100 _ 2304
123gr _ Lapua _ Scenar __ 0.55 _ 2900 _ 2297
140gr _ Berger _ VLD ___ 0.61 _ 2550 _ 2021
110gr _ GSC _ SP ______ 0.53 _ 3100 _ 2347
120gr _ GSC _ SP ______ 0.62 _ 2850 _ 2164

slimjim

fnuser wrote:

You are right in saying the good old .270 Win is hard to beat that's what made it so good and old.

A good way to state it!

fnuser

The kicker is the c.o.a.l. of 2.25" everytime, it always comes back to that doesn't it? I've been cruising other forums reading posts, and the owners of 6.8's are still trying to use 130's and are sorting magazine brands by their internal lengths. The one's that use a lap seam restrict the length by the thickness of the sheet metal, but to me it would be a better joint then a butt weld. Always a drawback or tradeoff. That lighter Barnes bullet might have some merit in this cart but for mass quanity and economy the sierra 110 might be the one depending on the volume of shooting and pocketbooks.

slimjim

I think the 130 gr bulelts are too heavy for the case size. Supposedly the PRI mags allow you to load up to 0.1" longer which is significant.

slimjim

I'm need to be cautious about achieving the BRX velocities listed in the chart. When reading the forums, most shooters are achieving those velocities with 28" to 31" barrels. The AR Performance company in CO has designed a bolt specific for the BR and BRX case made of stronger steel than standard AR bolts. They are the ones achieving these numbers. They also are using different primers (CCI 450's for an added 40 to 50 fps) and hand-lapped barrels (another 60 to 90 fps) along with 60,000 psi to achieve these velocities. I'm sure they are using Lapua brass which is very high quality.

I've studied the fire-forming process to make the BRX case and don't see that to be much of a challenge. I'm now starting to define min twist rate for each bullet as I try to develop a rational for which caliber to select. I catch myself wavering toward the 6mm but went into this wanting the 6.5. I want to make sure I don't end up with too much bullet for the case.