Muzzle Velocity in a Semi vs Bolt action

I chrono'd my Ruger Mini 30 (7.62x39) at the range today alongside a CZ bolt action in the same calibre. Did it across several ammo types including handloads. The bolt action shoots pretty much advertised muzzle velocity as per the packet (say 2460 fps for PPU / Prvi. The Ruger semi however, 1600 fps give or take). Now 800-900 FPS difference to me seems like something is amiss. From looking online, advice seems to be that there would be a difference (gas cycling etc) but not significant.

800 fps difference seems an awful lot to me, so do I just need to accept this or is this an indication something is amiss? There was little difference between being suppressed vs unsuppressed on the semi. The barrel length is about 30mm shorter than factory due to removing the foresight and adding a thread.

Any insight would be welcome!

Cheers, Feral.

A 30mm shorter barrel should only loose you 100fps at most, more likely around 50fps. The other thing I found with an F1 chrony was that my mates short barrel rifle it wouldn't read properly, possibly because it threw a bit of unburnt powder or debris out with it that messes with the readings. Try moving the chrony out another few meters to see what happens. What kind of chrony is it?

Barrel length has a big influence on mv, short equals slower long equals faster within reason. If the brno had a longer barrel than the ruger it would be faster. Bore dimension also affects speed (looser less friction faster). Rule of thumb is quoted mv is usually from a 22 or 24 inch barrel and you loose 50-100 fps per inch below that. Dont know the effect of tapping gas before the muzzle but it must account for a few fps. My 2c worth.

Thanks gents, shooting a 17" barrel, so that might say a lot. Although I'll need to measure my mates CZ as it looked not too dissimilar.

Using an F1 Chrony also, and I'm struggling to get it far enough forward. That said, the readings were consistent and similar whether the DPT was on or off the barrel. The CZ was unsuppressed.

Hmmm, nope, the CZ is basically the same barrel length... 17.5". Any other ideas?? Surely I can't be losing 800fps through the gas system. I'll shoot again tomorrow with a larger gas bush and see if there's any increased loss. Barrel cleaned tonight also so will see if there's any great change. Not expecting it.

Possibly very generous chamber dimensions to help reliable cycling with many types of ammunition (cheap military crap etc) and maybe loose bore dimensions also would account for a large drop in pressure and velocity.

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Originally Posted by veitnamcam

Possibly very generous chamber dimensions to help reliable cycling with many types of ammunition (cheap military crap etc) and maybe loose bore dimensions also would account for a large drop in pressure and velocity.

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Possibly. I'd be surprised if its the bore itself. Ruger went from .308 to .310 and thickened the barrel in the mid 2000s due to accuracy concerns. I'm shooting .310 but I might try a .311 jacketed or .312 unjacketed.

It is a large difference in velocity.

Can you get the velocity back loading up without reaching pressure signs?

Yeah that's what concerns me - the big gap in velocity. I'm already at the max as per the manual but marginally below factory speeds. No pressure signs. ADI 2207 at 26.5 gn pushing a 123 gn horn SP. Cases are once fired, trimmed & crimped.

I think I'll look at some .311 speer or sierra at 150gn. Bore is .310 not .308 so I should be good, might seal the bore better (if that is the issue).

I have cronyed both a 12.5" 7.62x39 saiga and a 16" bikal single shot-both with the same can and found there was only about a 250 fps difference using norinco cheep stuff but the Es was about double.
This is from memory anyway

The Ruger tube probably has a true .308 bore as opposed to .311 for the calibre. Shorter barrel (presumably) + smaller hole = more friction.

Smaller hole should put pressure and speed up.

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the difference in speed should mean way more drop, how does it compare in that regard?

This doesn't sound right. Energy loss in center fire semis is usually unreadable.

The only other thing I can think of is that the gas bushing. Change I did has led to the block not sealing as it should so I might be losing more gas than i realise. Will load up and have another shoot today.

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Problem solved. Too close to chrony so it must have been reading gas or powder interference. 2160 fps avg is much more like it.

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Yeah i had an f1 once, purchased one of these:

http://www.amazon.com/Competition-El.../dp/B0028MTBJ4

Problems solved

Our pistol club purchased an F1, needless to say it no longer gets used.
We found it didn't like being used anywhere near the shelter over our seating area,

And funny enough we all now use the Chrono linked above, as owned by a club member

I think the mini 30 has a 308 barrel not a true 312 like 7.62x39 should be. They have done that so you can use 308 projectiles when reloading but it could also be effecting the velocity. I agree with everyone else though - the chrony is giving you false readings.

Slightly off subject but I use 308 projectiles in my 7.62x54 nominal bore .310 same as the m43 7.62x39 way better selection and no drop off in accuracy or velocity I can measure unless using boat tails.

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Originally Posted by MassiveAttack

I think the mini 30 has a 308 barrel not a true 312 like 7.62x39 should be. They have done that so you can use 308 projectiles when reloading but it could also be effecting the velocity. I agree with everyone else though - the chrony is giving you false readings.

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Very first one were 30cal hence Mini 30.
http://www.brownells.com/GunTech/Rug....htm?lid=15904

Quote:

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Originally Posted by MassiveAttack

I think the mini 30 has a 308 barrel not a true 312 like 7.62x39 should be. They have done that so you can use 308 projectiles when reloading but it could also be effecting the velocity. I agree with everyone else though - the chrony is giving you false readings.

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The earlier mini 30s had a .308 barrel which had a smaller outside diameter also. Recent models like mine are .310 internal.

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My F1 is fine, follow the instructions.
I always extend it to the max length of the cord attached to the panel (around 12'). I always use the cover in sunny weather.