75 grain Speer Gold Dot gas drive

[Defaultmp3]

Some two more data points for 75 gr Gold Dot as measured by a MagnetoSpeed V3:

• Centurion Arms 14.5" with a SureFire SOCOM556-RC: 2611 FPS
• Centurion Arms 11.5" with a SureFire SOCOM556-RC2: 2471 FPS

Not sure why my numbers on the 14.5" are so different compared to the 16" and 20" data mentioned above.

[MistWolf]

How does this work? If an unusually slow burning powder is used why isn't there more difference in velocity between 16" and 20" barrels? What else would increase peak pressure or duration at the gas port compared to other 70+ grain loads?


https://www.youtube.com/watch?v=QswX2r6PcxM

A slower powder reaches peak pressure later, so it tends to be at a higher pressure when it reaches the gas port. Higher pressure at the gas port means gas flow through the port is increased which in turn, increases carrier velocities. Increased carrier velocities give sharper recoil because the reciprocating mass is hitting the back the the RE harder.

A 75 gr bullet is heavy for the caliber and longer. A slower powder is needed to accelerate the bullet to best velocity without the pressure spiking too quick. Heavier bullets need a slower initial push but need the pressure to keep building longer to get them going.

However, case capacity limits how much of that slower powder can be used. Case capacity is further limited by the long bullet needing to be seated deeper in the case in order for the ammunition to fit in the magazine and the throat. While the pressure peaks later, causing higher carrier speeds, it's not enough fuel to keep accelerating the bullet from longer barrels.

A 75 gr bullet launched from an 8 pound rifle generates 3.75 ft/lbs of free recoil. a 55 gr bullet launched at 3100 fps from the same is 2.74 ft/lbs.

[Disciple]

However, case capacity limits how much of that slower powder can be used. Case capacity is further limited by the long bullet needing to be seated deeper in the case in order for the ammunition to fit in the magazine and the throat. While the pressure peaks later, causing higher carrier speeds, it's not enough fuel to keep accelerating the bullet from longer barrels.

I am hung up on this part. If the 75gr GD uses a slower burning powder than 75 to 77 grain 5.56 NATO loads shouldn't it show more velocity difference between short and long barrels than 5.56, rather than less? Pressure doesn't just fall of instantly, does it? Could the 75gr GD have a secondary pressure spike that is happening close to the gas port? A Lucky Gunner article shows a second pressure wave from Tula for example.

[MistWolf]

I am hung up on this part. If the 75gr GD uses a slower burning powder than 75 to 77 grain 5.56 NATO loads shouldn't it show more velocity difference between short and long barrels than 5.56, rather than less?

Not if there's not enough fuel (powder) to keep accelerating the bullet. Remember, there is less powder driving the 75 gr bullet than the 55 gr.

One example this phenomena is the .22 rimfire. There is little to no velocity gain after about 18 inches and in fact, much longer than that, a .22 rimfire will start losing velocity due to friction.

Pressure doesn't just fall of instantly, does it?

Correct. Pressure tapers off.

In fact, the pressure in the bore does not drop instantly to zero when the bullet exits the muzzle. It takes time for the residual gas to blow down. It's the residual gas in the bore that powers the action.

Could the 75gr GD have a secondary pressure spike that is happening close to the gas port? A Lucky Gunner article shows a second pressure wave from Tula for example.

That's very possible.

[Disciple]

Not if there's not enough fuel (powder) to keep accelerating the bullet. Remember, there is less powder driving the 75 gr bullet than the 55 gr.

One example this phenomena is the .22 rimfire. There is little to no velocity gain after about 18 inches and in fact, much longer than that, a .22 rimfire will start losing velocity due to friction.

The 5.56 75 and 77 grain loads are working with the same powder volume? The numbers I can find for heavy 5.56 show a greater increase between 16" and 20" than the 46 fps from the video I quoted for 75 grain Gold Dot. One set of MK262 values attributed to Wes at MSTN shows a 149 fps gain between 16" and 20" for example. Do you think one of these is an error or fluke?

[Defaultmp3]

I was not expecting such a big drop in velocity between my 10.3" and 11.5", but my 10.3" Centurion Arms barrel with a SureFire SOCOM556-RC2 was chronoing at 2245 FPS, over 200 FPS slower than my 11.5".

[1168]

I don’t have a real answer, just some long winded observations.

I think the slower bullet may actually increase gas drive, in a way, by increasing the amount of time that it is in the bore after passing the gas port. Combined, of course with whatever powder they are using. I’m certain that that isn’t true all of the time, like, you can’t just reduce powder charges and expect more gas drive. But whatever powder they are using seems to have a lot of drive with that slow bullet, and a presumably light charge. There seems to be some voodoo place where timing and gas volume line up, though.

I’ve been experimenting a bit with a pair of Colt 14.5” barrels. One is choked down to the point of unreliable function. We’ll call that barrel B and the stock one barrel A. Barrel A will eat M855 and 62gr/WC846 max loads (2875/14.5, like 30-35fps slower than 855) suppressed or unsuppressed with no drama. It also eats pretty much whatever, honestly. Barrel B will not reliably cycle, nor will it lock back, ever, with either of those loads, unsuppressed. Add a can, and it eats them drama free.

But here’s where it gets weird: remember that Precision One 62gr 556 I chronoed a couple of years ago going like 350 fps slower than it should? Well, it doesn’t cycle unsuppressed, yet is crazy gassy with a 308 can on barrel B. So gassy, it blows gunk out the gap between the receivers onto my left arm. Filthed up the outside of the mags, horrifically. Its only moving like 2400 fps. The same gun has no drama at all shooting pretty much anything else suppressed.

Another strange thing is when I tried FGMM. Its .223 pressure 69gr that is loaded very light. I’ve not cronoed it in a while, but people get like 2550 ish from it in a 14.5” barrel. Its really weak stuff that tends to provide good precision. Somehow, in Barrel B, which will not reliably cycle unsuppressed with any other load I’ve tried, including some fairly hot 62gr and 68gr loads, this stuff cycled 100% and locked back on empty 60% of the time. In other words this slow ass .223 load cycles a gun that won’t run with M855 or a hot 68gr. Why might that be? I think its that voodoo where timing (slow bullet) and pressure peak and gas volume coincide with the gas port location somehow.

TL;DR: this stuff is interesting, but all I have is more food for thought. I’ll try CFE soon in that barrel… it’ll cycle most anything with a starting load. If I can find any GD 75, I’ll try that, also.

Edit: Barrel B cycled 100% with 26gr CFE223 /62 gr. This is nowhere near max, but CFE 223 is a pretty slow powder.

[Disciple]

Thank you. Your tests make it plain that if you throttle the gas you should test the gun with any load you might need to rely on.

[1168]

Thank you. Your tests make it plain that if you throttle the gas you should test the gun with any load you might need to rely on.

Thats probably the most reproducible conclusion that could be gleaned from my rambling.