Rem 700 SPS w/ 16" barrel wildly erratic

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oldscot3

New member
As mentioned in a couple of other posts, a hunting buddy and myself both bought suppressor ready 308s recently, mine a Howa and his a Remington SPS. I've been working up subsonic loads for both guns using Sierra 220 gr. round nose bullets and Trail Boss.

We've been twice to range to chronograph the loads in our guns. On the first outing my Howa with a 20" barrel was just over 1000 fps while his was substantially lower.

On the second outing with powder charges increased by 1/2 gr. increments, my Howa came predictably to about 1100 fps. When we put his gun on the bench though, the head scratching began. The first shot in the 800s, slower than what we had shot on the first outing. The next shot jumped some 1000 fps to just over 1800 fps. This kept happening, so our thought was something must wrong with the chronograph.

We took his rifle down, put mine up... good, predictable results were back. We also shot a 22 Hornet across the screens and got good repeatable data. Then we put his gun back on the bench and the previous results repeated themselves. So I thought there must be something about our subsonic handloads that his gun really doesn't like. To test this theory we fired two shots of PMC 150 gr. factory loads. The first shot registered 2734 fps, about right I thought, but the second shot clocked 3187 fps. Wow what a muzzle blast out of that stubby barrel!

I'm at a loss, what could be wrong here? Something causing radical pressure increases? Could it be a problem with the chamber grossly out of alignment with the bore? Any other ideas?
 
just AWAG but perhaps the muzzel blast is interfering with the chronograph. Try setting the chronograph farther from the muzzle and check it again.
 
No way you got almost 3200 fps with that load, even in a 26" barrel. I've seen some hot 150 gr handloads reach 3100 fps or a touch more from long barrels, but not with factory loads. At least with that shot you had a chronograph error.

I'm don't know enough about reduced sub sonic loads to say much.
 
No way you got almost 3200 fps with that load, even in a 26" barrel.
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I agree, one would think to produce that velocity pressure would have had to be very, very high and the bolt lift would be hard. There was however, noticeably higher blast from that shot. The unexplained thing still is, how come I'm only getting chronograph errors with the Remington?

The idea that muzzle blast was effecting our readings is a nice guess, but the chronograph was set up 3 yards from the muzzle and we had an even wider speed variation with the subsonic loads, with the Remington, which were producing very little recoil or muzzle blast.
 
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My buddy's idea is to buy a box of Remington factory loads and run them through both of our guns. Then, if the same thing happens, call Remington and see if they suggest returning the gun.

I guess we could set up the chronograph even further away.
 
First,I don't know!

I'd suggest moving the chronograph a little farther than three yards.Try five,just to see.

Given what you are exploring for loads(lowvel,low pressure) ignition and burn are issues.That can be the obvious : primers,crimp,if any,etc.

But you are also dealing with two barrels.Leade,throat,throat and bore erosion,rifling twist and configuration,and bore dimensions will all influence a bullets resistance to going through the bore.At low pressure loads,that might make a big difference in burn. Heck,copper fouling could be the problem,and a good cleaning might be the answer.
Slugging the bores might reveal something.

At the velocities you want,have you considered cast bullets? .One rifle might prefer .002 larger bullets,for example.
 
Thanks for the suggestions...

I'm going to have to toss out copper fouling as it's a new gun with fewer than 20 rounds.

When we test with the Remington factory loads we'll definitely move the chronograph out further. I really hope that's all it is.
 
I'd have said chronograph wildly erratic, not barrel. I set my chronograph up no closer than 15 feet from my muzzle, if I'm shooting a muzzle loader I move it back to 20 feet. I rarely get any errors unless the lighting is all wrong.
 
I'd have said chronograph wildly erratic, not barrel.
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I suppose, however, the chronograph wasn't wildly erratic except with the Remington and its 16" barrel. At the range I didn't think the chronograph was too close since we were shooting bunny fart loads but I'm going to heed all the good advice here, move it back way back and try again. Honestly, I've been afraid there was something wrong with the rifle that was driving pressures up dangerously. I'll be relieved, as will my buddy, to confirm it's just our error in setting the chronograph too close.

I'll post the results in a couple weeks after my friend can everything back out to the range for another test.
 
Have somebody check the chamber. The throat may be short, pinching the bullet in the neck. I'd be cautious of that rifle for sure.
 
Good idea. So far we have only used new Winchester brass,(with the exception of the PMC factory load mentioned) but if the throat was short as you suggest may be a possibility, perhaps every other case was just long enough to cause a pressure rise.

I've got 30 cases we haven't used yet , if I measured them all and trimmed them all to the length of the shortest one, that might tell us something if the problem goes away.
 
My guess is that you're not burning all the powder in your buddies shorter barrel. That powder is being propelled faster than the bullet and is interfering with your chronograph. If the ammunition isn't giving you issues when chambering I seriously doubt there is anything wrong with the chamber.
 
All testing so far is without a suppressor on board. It's still in suppressor jail.

The goal right now is to get our sub sonic loads worked out for both guns to ensure stability. My Howa is pretty well dialed in, but we're having this problem with my buddy's Remington.

The powder is Trail Boss, we'll definitely move the chronograph back as a first step to see if data settles down and seems reasonable.
 
The 16" barrel is not burning the powder. You are chronographing burning powder being pushed by the gas. Put the Chronograph much further away from the muzzle.
 
I'm gonna go with the move the chrony back crowd. But my biggest question is will a 1/10 twist barrel stabilize a long 220gr bullet at that low of a velocity?
 
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