308 Interlock to SST = +150 FPS

[Ike Clanton]

My original load was 50 grains 2000MR with Hornadys 150 Interlock flat based that chrono’d 2600. Recently picked up an SFAR and loaded some SST’s instead. Same load, brass and primer but it measured 2750 FPS. I double checked the interlocks and they still shot at 2600. What gives? More than happy with the increase however.

[44 AMP]

Tbe clear, are you talking about two different bullets?? Fired from two different rifles???

[Shadow9mm]

Originally Posted by 44 AMP

Tbe clear, are you talking about two different bullets?? Fired from two different rifles???

Thats what it sounds like to me. 2 different rifles, possibly 2 differentbarrel lengths, 2 different bullets, using the same powder, brass, primers. Im guessing 308 by the sfar reference

[Ike Clanton]

No same Rifle 16 inch. Now that being said I have fired the interlocks from a different 16 inch with the same fps. As well as some factory loads. So nothing has changed but the bullet.

[Ike Clanton]

I should also mention the SST load was once fired brass and the interlock was virgin but all the same batch of Winchester brand. I can’t imagine that would have an affect. All shots had perfect 3 o’clock ejection and no pressure signs. I chrono’d a few loads at 51 grains and got 2800 but began seeing ejector marks. Hornady’s max is 51.2 grains and Speers is 52.8.

[jmr40]

I have seen 2 different rifles with the same barrel length shoot ammo from the same box 130 fps different. That is on the high side, but 25-50 fps difference with the same length barrel on different rifles is common. I've never seen 2 different rifles with the same length barrel be exactly the same. Close yes, my 3 rifles in 308 are within 25-30 fps. My two 30-06 rifles are 60-90 fps different depending on the load.

Never used that powder, but I have used Alliant powder in the past. Some of it is known to give unpredictable velocities. I was loading a 300 WSM with RL17. While still 2 gr below max I hit a velocity spike and was 50-100 fps faster than it should have been. I've read of others seeing things like this with some Alliant powders. But once again, no personal experience with 2000MR

If I'd not had a chronograph and loaded up to book max I'd have been quite a bit overloaded with the components I was using in that rifle. Maybe not in another rifle.

Just a guess; while the bullets weigh the same the SST's shape might be different enough to cause pressures to be far more than you got with the Interlocks.

BTW, 2750 sounds too fast from a 16" barrel. While you aren't seeing pressure signs, you may still be overloaded

[Ike Clanton]

That was my concern was the velocity. By my calculations I should be in the 2650 range. I’m gonna shoot another string and make sure it wasn’t a chrono error. With factory ammo this rifle appears to be 50 FPS faster than my other 308 16 inch so I need to figure out where the extra 100 fps is coming from.

[USAF Ret]

Interesting conversation as I have a Ruger SFAR. It's a 20 inch barrel and I plan on loading some 150 SSTs with Varget.

[Ike Clanton]

Great let me know your experience. Lots of info on Varget loads. Though it’s seems like lots of peoples loads go way over the listed maximums with that powder.

[44 AMP]

Hornady’s max is 51.2 grains and Speers is 52.8.

In their test guns.

So, same everything except the bullet....
differences in bearing surface and jacket hardness would account for different velocities.

SO, what else might be different?? How about your chronographing setup?

Shooting outdoors, using natural light, right?? same angle of the sun? (time of day, time of year??) same intensity of sunlight?? (clear, overcast, etc??)
Ambient temp the same??? (or roughly?) Same distance muzzle to chrono?

Even how the rifle is held may make a measurable change to chronograph readings, lots of things can affect the results.

Most likely explanation to me is that the SST bullets just "slid down the barrel a little easier" which resulted in an increased velocity reading, compared to the same load with a different bullet. Not super common to have as much of a difference as you saw, but it happens sometimes.

[Paul B.]

If I get this right one bullet is a spitzer flat base and the SST is a spitzer boattail. Could it be that the bearing surface of the SST is a bit less that the flat base bullet, thus there being less friction to slow the SST down?
Paul B.

[Ike Clanton]

That’s about what I can figure. I went out today and shot another string and made sure my chrono was set up perfect. I was able to get a more accurate spread but the SST is still 90 FPS faster. While doing so I discovered my brand new Leupold VX5 3-15 has a split reticle. Back to the factory it goes.

[Unclenick]

No, it's not reduced friction. This is counter-intuitive, but if you reduce friction, you actually lose velocity because reducing friction lets the bullet pick up speed and start expansion of the space behind it earlier in the powder burn. As a result, peak pressure happens in a larger volume of space and goes down. When moly-coating bullets first became a craze, people kept getting annoyed that their favorite load lost 20-50 fps, and they had to add a little powder to get back up to speed.

Several things come to mind, some of which have already been mentioned:

Chrono setup. SAAMI testing standards use an optical chronograph with the midpoint between screens at 15 feet from the muzzle. Two reasons to match that are:

1) To match what's published, and

2) Lots of folks get high readings or low readings that can be remarkably consistent (but wrong) due to debris like unburned powder flying out of the muzzle. A short barrel turns more of that stuff loose than a long barrel, so if you've been closer to your chronograph than that, it could well be part of the problem.

The chronograph is set up on light-colored soil or sand rather than grass on a bright day. This can cause false velocities due to the sensor picking up a light reflection off the ground. Spread a dark tarp out under an optical chronograph if you have this condition.

If the sun is anywhere near being overhead, make sure your sky screen diffusers are shading the sensors from direct sunlight and that their shadow isn't off to the side or in the middle somewhere. In some conditions, particularly if the sun is not directly overhead, some sky screens actually like the sky and work better with the diffusers removed, so you can try this if the sun is at an angle.

If all that is copacetic, then we get to higher pressure. Higher pressure is caused by several things. One is the longer bullet seating deeper into the case, thereby giving the powder less room to burn in. I did this in the GRT software. Alliant's specs say 51.5 grains should produce 2950 fps from a 24" tube using a Speer 150-grain BTSP. I plugged that load into GRT and found that with the case capacity set to 57.91 grains, I got a velocity match at reasonable pressures. Changing the bullet to the SST and keeping the 2.800" COL, I got 2994 fps, and the pressure went up from a peak value of 56,999 psi for the Speer bullet to 61,444 psi with the SST, or almost 8%. With a 16" barrel, the velocities drop from 2950 fps and 2994 fps to 2657 fps and 2709 fps, respectively. So the bullet choice is making a real difference here, but not 150 fps all by itself. I am going to guess the bullet's ogive is also closer to the rifling lands, which can also raise pressure like this and may well account for the rest of the difference. In any case, pressures are higher with this bullet, and I would back the load off 5% and see how that flies.

[Ike Clanton]

Unclenick thank you so much for the thorough response. I’m always learning here. I appreciate all you guys.