Help troubleshooting groups

[Coejro]

Brief background, I have a mid 90’s Remington 700P in .308. Rifle sits in a KRG bravo chassis, with an SWFA 10x42 side focus.

I’ve been able to dial the rifle in to ~.75 MOA groups with Federal Gold Medal March 175gr Sierra March King HPBT’s.

On a whim, I tried out a box of Hornady ELD-X in 175gr, and averaged 3 MOA groups.

Thinking I messed something up with the scope or zero, I switched back the the FGMM and shot a .75 MOA 5 shot group. Back to the Hornady’s and the grouping opened back up again.

What gives? I’m not really concerned with the ammunition, more just curious what may be causing the variance with same weight bullets.

Groups posted below:

 

[ShtrRdy]

I don't know what it is about the ELD-X bullets but I didn't have as good of groups either. The other part of this is your rifle just doesn't like the Hornady ammo.

 

[Precision Underground]

Different ammo will almost always produce different results. Some results will be better, some will be much worse.

 

[Dougie308]

What he said. My modified 1995 700 VS shot roughly 1" groups at 100 yards (from bags and a bench rest) with 168 gr Federal Gold Match. With 168 gr Hornady Amax I can shoot .5" (again from bags and a bench). But with 168 gr Powershocks it goes to about 2". Microscopic faults in rifling, variances in the powder charge or the manufacture of the bullet all become greatly exaggerated when you have an object travelling 2700 FPS over 100 yards.

Different ammo will almost always produce different results. Some results will be better, some will be much worse.

 

[Coejro]

Ok, that makes sense. I was surprised by how much variance there was, and wanted to make sure it wasn’t something out of the norm.

 

[AllWorkAndNoPlay]

I’ve seen similar results from Hornady 147 ELD ammo in 6.5 CM in a 1:7.5 twist 26” Kreiger barrel. I did a measurement of the neck of that loaded round and found it to average 0.289” which is 0.006” tighter than the chamber spec for a 6.5 CM and my once fired brass necks measured on average 0.295” which is right at spec., so it seems to me that neck tension may be causing my large groupings.

In order to test this theory I plan to hand load some Hornady 147 ELD bullets using the same brass, length, same powder & charge they used to a neck size of 0.294”.

 

[FishDr]

I’ve seen similar results from Hornady 147 ELD ammo in 6.5 CM in a 1:7.5 twist 26” Kreiger barrel. I did a measurement of the neck of that loaded round and found it to average 0.289” which is 0.006” tighter than the chamber spec for a 6.5 CM and my once fired brass necks measured on average 0.295” which is right at spec., so it seems to me that neck tension may be causing my large groupings.

In order to test this theory I plan to hand load some Hornady 147 ELD bullets using the same brass, length, same powder & charge they used to a neck size of 0.294”.

Wow - .006 difference in the outside of the neck when loaded? What changed there? Is this same brand of brass? I would think when the bullet is pressed in place, the OD would be very similar regardless of neck tension. We’re these different brand, necks turned ? Something isn’t making sense to me. Not being rude, trying to understand the situation. Thanks.

 

[AllWorkAndNoPlay]

Wow - .006 difference in the outside of the neck when loaded? What changed there? Is this same brand of brass? I would think when the bullet is pressed in place, the OD would be very similar regardless of neck tension. We’re these different brand, necks turned ? Something isn’t making sense to me. Not being rude, trying to understand the situation. Thanks.

It really doesn’t make sense, but as you can see in the pics below it’s a fact. New factory Hornady ammo...

 

[FishDr]

You’re not measuring neck tension there. That’s chamber clearance / bullet release. Be careful if you mix chamber dimensions with cartridge dimensions. If you size that case back down, regardless of neck tension, and squeeze a bullet back into it… it will be back to that .289 diameter.
That said … if the chamber neck is too large (many factory rifles), it does affect accuracy potential.

 

[AllWorkAndNoPlay]

You’re not measuring neck tension there. That’s chamber clearance / bullet release. Be careful if you mix chamber dimensions with cartridge dimensions. If you size that case back down, regardless of neck tension, and squeeze a bullet back into it… it will be back to that .289 diameter.
That said … if the chamber neck is too large (many factory rifles), it does affect accuracy potential.

LOL, and I try so hard not to be confused or conflicted...

I'm not trying to steal OP thread, but just to to clarify my neck tension calcs are based on the fact that a new Hornady 0.264" dia. 147 ELD bullet for reference will not go into a once fired 0.295" Hornady brass neck without being pressed in, so starting there the bullet will already have some unkown amount of tension on it then add on the 0.006" of tension down to their loaded neck size of 0.289" and you get a least 0.006" of neck tension with their factory ammo and it's probably more...

OP, I'll let you know if this tension is the culprit or not once I have the results of my little experiment. Additionally, these factory loads are loading at an ogive length that is a 0.070" jump to my lands, so I'll be experimenting with seating depth too in a different experiment, if neck tension turns out not to be a factor.

 

[FishDr]

Necks can only get bigger when you press in a bullet. They can’t get smaller. Measure the OD of the brass after firing (largest), after sizing but before bullet seating (smallest), and then after seating the bullet (middle dimension). The difference in the last two measurements approximates your neck tension. Difference in first and last measurement is your bullet release (approx).

Edited to fix autocorrect spell errors.

 

[AllWorkAndNoPlay]

Necks can only get bigger when you press in a bullet. They can’t get smaller. Measure the OD of the brass after firing (largest), after sizing but before bullet seating (smallest), and then after seating the bullet (middle dimension). The difference in the last two measurements approximates your neck tension. Difference in first and last measurement is your bullet release (approx).

Edited to fix autocorrect spell errors.

Thanks Captain Obvious!

 

[FishDr]

No problem dude. Just trying to help out with your .006+ neck tension measurements. That much wouldn’t quite deform shoulders, but would be really stiff seating. Drive on and enjoy.

 

[Coejro]

OP, I'll let you know if this tension is the culprit or not once I have the results of my little experiment. Additionally, these factory loads are loading at an ogive length that is a 0.070" jump to my lands, so I'll be experimenting with seating depth too in a different experiment, if neck tension turns out not to be a factor.

Much appreciated. I need to get a pair of calipers and check the rounds I have as well.

 

[AllWorkAndNoPlay]

Another thing is that most all 308 barrels like the plain vanilla 175 gr SMKs and they can easily be worked up to make very tight groups as show below outta my 24" Bartlien barrel:

 

[AllWorkAndNoPlay]

Hornday 6.5 CM 147 ELDs Test result:
Pulled bullets from 10 Hornady factory loads and measured neck ID at 0.263" so factory neck tension isn't an issue and it is about perfect.
Measured powder and found it was just about perfect too at 44.0 gr +/- 0.1 gr. for all 10 test rounds.
The length to Ogive varied about 0.009" but that's not a problem either since in my rifle they work out to a 0.065" jump, so I did a jump test by hand loading these 10 cases back up with 44gr of the original Superperformace powder and reseated the bullets to a 0.026" jump, which was about as long an OAL that my mag would hold.

As you can see below the shorter jump makes a little better group, but what helped me most was shooting a hot & dirty barrel for both groups cuz these groups are way better than what I got when I shot factory loads with a clean cold barrel. I've decided that my barrel just wont shoot the 147 ELDs any tighter than this so I will be working up loads with 150 SMKs and Berger 140 Hybrids to see if my groups tighten up any.

 

[FishDr]

In order to test this theory I plan to hand load some Hornady 147 ELD bullets using the same brass, length, same powder & charge they used to a neck size of 0.294”.

Did you have any better results from this combo??