Federal GMM 260 Remington Conundrum

[Legionnaire]

I have often shot Federal Gold Medal Match ammo to set a baseline when checking the accuracy of a new rifle. I have had good success when shooting .308. But I was disappointed yesterday. Weather conditions were not great for shooting groups (a bit windy) but I figured I could get sighted in ... and I did.

But the velocity data for the GMM in .260 Rem was weird. I shot a box of 20 using my LabRadar. In that box I had two rounds that clocked well over 3000 fps: 3270 and 3999. With those included in the string, my numbers were:

Average Velocity : 2857
Extreme Spread : 1294
Std Deviation : 292

Excluding the two aberrant shots, the numbers were:

Average Velocity : 2771
Extreme Spread : 102
Std Deviation : 23

That second set looks better, but the ES is still higher than I'd have thought.

I was shooting at a pace of one round every two minutes to avoid overheating the barrel. Temps were in the low 70s, and though the barrel warmed, it never got hot to the touch. Gun and ammo were in the shade the whole time. The first high velocity shot was shot number 6; the second was shot number 18.

Anybody else experienced this? Any possible explanation other than an overcharged or maybe an out-of-spec case? I have on occasion had the LabRadar fail to read a shot, but don't know that I've ever had a significant misread before.

 

[Goosey]

142 @ 3999 fps provides the energy of a 338 Lapua or 470 Nitro Express, being only a 260 it's fair to assume it's simply a chrono error.

 

[Bfh_auto]

My 260 won't even push 100's that fast. You would have been beating the bolt open with a mallet or worse to reach those velocities.

 

[Legionnaire]

Yeah, that makes sense, at least the 3999. Gotta be a read error. I'm still getting the hang of the LabRadar. Doesn't bode well if it gives inconsistent readings.

But excluding the two 3000+ plus readings, does a 102 fps extreme spread sound reasonable for match ammo?

 

[Bfh_auto]

Yeah, that makes sense, at least the 3999. Gotta be a read error. I'm still getting the hang of the LabRadar. Doesn't bode well if it gives inconsistent readings.

But excluding the two 3000+ plus readings, does a 102 fps extreme spread sound reasonable for match ammo?

I haven't shot much factory match ammo, but most of it was under 50 fps ES. You may have gotten a bad batch, but I would hazard a guess that your Chrono was having a bad day. Especially if it shot well.

 

[GooseGestapo]

Sounds like you got a signal reflection or some electrical interference that generated an erroneous reading.

When I was a police officer running radar in early ‘80’s, I could point an MPH radar unit (X-band) at a TV cable box on a power pole and get a 62mph reading. Going out to the airport and driving past the end of the runway supporting an ILS system, passing under the localizer beam would give a 98mph reading!

Radar chronographs have their own quirks.

 

[Varminterror]

Yeah, that makes sense, at least the 3999. Gotta be a read error. I'm still getting the hang of the LabRadar. Doesn't bode well if it gives inconsistent readings.

But excluding the two 3000+ plus readings, does a 102 fps extreme spread sound reasonable for match ammo?

1) Definitely a read error at 3999.

2) 102 FPS ES might not be excessive for certain types of short range matches, but for long range shooting, it’s way, way too much. Not so uncommon to see that in factory match ammo, however. At 1,000yrds, 102fps spread can be more than 20” of unresolvable vertical dispersion. Not OK.

Here’s a difference of 24ES vs. 78ES at 875yrds. The bottom 3 on the left hand target were sighters, then I corrected .2mils, and sent the group. The right hand target was a group from a 6.5 Grendel shooting Hornady Black Match 123 ELD-M’s with 78fps ES typical for that lot. The left hand target was reloaded 105 Hybrids in 6 creed with an ES of 24 for that batch. Those are 66% IPSC’s, so the body is 12” wide by about 16” tall (less the head). Both loads were .6-7moa at 100-200yrds. You can see the difference in vertical dispersion. Both groups are spread in the wind about the same, just about 1moa wide, but the lower ES load pumped out a sub-MOA stripe, while the high ES load scattered a tall column greater than 1.5moa.

 

[BigBore44]

1) Definitely a read error at 3999.

2) 102 FPS ES might not be excessive for certain types of short range matches, but for long range shooting, it’s way, way too much. Not so uncommon to see that in factory match ammo, however. At 1,000yrds, 102fps spread can be more than 20” of unresolvable vertical dispersion. Not OK.

Here’s a difference of 24ES vs. 78ES at 875yrds. The bottom 3 on the left hand target were sighters, then I corrected .2mils, and sent the group. The right hand target was a group from a 6.5 Grendel shooting Hornady Black Match 123 ELD-M’s with 78fps ES typical for that lot. The left hand target was reloaded 105 Hybrids in 6 creed with an ES of 24 for that batch. Those are 66% IPSC’s, so the body is 12” wide by about 16” tall (less the head). Both loads were .6-7moa at 100-200yrds. You can see the difference in vertical dispersion. Both groups are spread in the wind about the same, just about 1moa wide, but the lower ES load pumped out a sub-MOA stripe, while the high ES load scattered a tall column greater than 1.5moa.

View attachment 837970

How do you weigh your charges? And to what decimal place? I’m asking because I weigh my charges to the hundredth. Even for my levergun hunting rounds. And I’m sure the type of shooting you do requires, at the very least, the same standard. Am I correct?

 

[Legionnaire]

Thanks for the feedback. As I noted, I am still getting used to the LabRadar.

I should have said earlier that I wasn't too happy with the performance of the 260 GMM either. It generally shot sub-MOA in my .308s, but it hasn't done very well in my .260s. Did shoot okay in one of my .260 Ackleys, though. But Saturday I wasn't doing much better than 1.5 MOA with that box of ammo.

Hand loads the next time out.

 

[Varminterror]

How do you weigh your charges? And to what decimal place? I’m asking because I weigh my charges to the hundredth. Even for my levergun hunting rounds. And I’m sure the type of shooting you do requires, at the very least, the same standard. Am I correct?

I do measure to the hundredth some of the time, but not because I need to.

According to my velocity curves and ladder testing, I could throw charges for that 6mm Creed barrel to within +/-.25grn (.5 grn spread) and still yield SD’s below 20, and ES’s below 45. Even the Lee Autodrum or Perfect Powder Measure is more than accurate enough. At my worst - for precision Rifle loading - I dispense on a Chargemaster which is only good for +/-0.1grn, at my best, I have an A&D and a Mettler Toledo which will go down to +/-0.015grn. In reality, I would only need to measure with such precision to ensure the validity of my work-up loads, and once the load is established within the node, a volumetric drum like a Harrels is plenty accurate enough to make ammo.

I really only use the analytical balances with the extreme precision because they are faster, not because they are more precise. The Auto-throw & Auto-trickle set up will dispense every 9 seconds, with every charge perfect, whereas the Hornady AutoCharge, RCBS Chargemaster 1500, or Lyman Gen5 will only punch out a charge every 30 seconds or so, and a handful out of 100 will need correction (easy to have a couple extra kernels of H4350 fall and add .1-.2grn). So instead of running two Chargemasters and the Lyman, I can throw more accurately, more precisely, just as fast with ONE set up. Of course, that ONE set up costs as much or more than those 3 others combined.