6BR Benchmark Loads

[gbal]

Do you mean that the heavy bullets still achieve closer to the muzzle velocities that they would achieve in a longer barrel,than do the lighter bullets in the same short barrel?

Yes that's what i seem to have found mate. May only be my rifle but thus far I can throw the really heavy bullets nearly as fast as the lighter bullets and not a million miles from guys using barrels much longer.

Danny,so you are talking 308(not 6BR as OP)?

 

Your 21 inch barrel has a muzzle velocity for 208 g bullets that is 'nearly as fast' as 175 bullets (can we assume equal pressures-otherwise it's all muddled-of course a heavy charge could make 208s faster than a much lighter charge for 175g bullets).

Do you have aactual accurately measured muzzle velocities for the two?

 

 

And the muzzle velocity for the 208 g heavy bullets in your short barrel "is not a million miles from the velocities others get with these 208 g bullets when fired in much longer barrels".Again,measured velocities,and same pressure loads?

 

Just to complete,do you think your 175 g bullets are very close in muzzle velocity to 175g from longer barrels too? Measured?

 

IF so,then one explanation might be that your short barrel is exceptionally ...err efficient,almost enough to compensate for the well established,if variable reduction in muzzle velocity with sub optimum barrel length- that optimum being up around 30 inches for 308?Barrels can,and do,give different velocities (again.we have to assume parity in powder used etc.)-but if we assume 20 fps slower per inch barrel shortening,that's quite a lot for the 'efficient short barrel' to compensate for,possible...but a lot may hinge on the assumption of equal barrel/load pressures,which can't readily be measured...though you may have data for your loads,and 30 inch competition 308s are unlikely to be underloaded,so will be close to max...

 

Interesting,indeed...

g

[dannywayoflife]

16" barrel mate

[gbal]

16" barrel mate

Well OK....16 inch just up's the ante-with 14 inches less barrel,we might be looking at well over 200fps typical degrade in MV.

The issues,Danny,are whether we are comparing like with like in the pressure/load department,and the only clues to that are actual MV and powder load detail ,and only you have that for your rifle.No pressure.

 

( a competition custom action Ackley hot load could well get 400fps more in a 6.5x55 than in a light factory load for old Krags,but there is a clear reason....powder and pressure).

g

[Jay1]

Do the barrels have the same twist? Same powder for both loads? Maybe faster burning powder best suited to a shorter barrel!....

 

I lost 230fps shooting the same load (155g) out of 20inch 1:12 twist .308 from my 30inch barrel

30inch - 3000fps

20inch - 2770fps

 

You might just have a fast short barrel, whatever it is you have to be quite pleased at the minimum loss of velocity.

[Jay1]

Does anyone use or have tried Sierra 107g SMK?? How do they compare to 105g AMax for target shooting?

[gbal]

Jay..see Accurate Shooter site for most things 6BR related,if you have not checked it out already.

There are several 100+g bullets recommended,and you will know what your 105Amax do- the 107 Sierra MK will probably be similar,but it's likely to be down to individual barrel preferences,and fine tuning.

 

Ditto Berger 105 VLD and Lapua scenar 105-only a decent test-10 shots?-in very good conditions will give answers for individual rifles....

 

gbal

[Jay1]

Load development is coming along nicely, if only I could get one or two calm days.

 

Both the 107gSMK and 105Amax shoot very well with 29.2g Benchmark. SMK seated 10thou off lands and AMax 3thou off lands.

I had them both out to 800yds and 1000yds last Sunday in extremely windy conditions - 12mins left wind @ 1000yds and holding left edge to right edge of board with pick ups/let offs, not much to be learned but great fun. I managed to get 7 shots off in the same condition @ 800yds with the SMK and they held easily under 1/2moa.

 

I'll be at it again this Sunday, hopefully get the chrony out too.

[Jay1]

After another day at the range I think I am finally finished, for now anyway!

 

I got the chrony out now that I have both loads delivering 1/4 moa

 

105g - 29.2 Benchmark, Base to ogive - 1.780, 0.003 off lands - 2810fps

87g - 32.5g Benchmark, Base to ogive 1.765, 0.010 off lands - 3255fps HOT LOAD!

XXXX WORK UP SLOWLY XXXX

 

The 87s shot 1.5 inch group @ 600yds yesterday with the above load.

 

I hope the above loads might serve as a reference to someone else looking to use Benchmark.

 

Also worth noting - I'm using a "cheapo" BERGARA barrel, for the price the accuracy they deliver has exceeded my expectations.

[onehole]

Been an interesting topic Jay and thanks for pm......I haven,t had chance to try Benchmark but will make some up just to check out velocity/pressure comparison with my normal vit140 and like you just to serve a reference for someone using 140 with a 26" tube I get a rounded up 2750,2800,2860 with 29.5,30 and 30.5,,,,,30.5 starting to max out due to my shortish freebore of only 60 thou{using 105Amax 5 thouoff} Interstingly all these loads can give repeatable 1/4 moa groups as long as I do my bit with the 30.5 probably being the pick but not an all year round load.My rifles sound different when they get close to "max" does anyone else notice this,,,?,,,,,,,,,O