Range Report .308 FGM 168/175 Temperature Effect on MV

[colson]

Would anyone like to share their data on muzzle velocity variations for FGM 168/175 due to temp changes? My range time is very limited. I'd like to see what others are observing. For example, my R700 24" 5R with FGM 168 was produced a measured 2575 fps (10 ft from muzzle) at 80 F, station baro 29.95. Seemed a little slow, but that mv input to JBM was putting me on target to 620 meters that day.

I've read everything I could find on this subject, but actual field data seems scarce.

I'd really like to see results from the extreme ends of the temp scale, 0 to 120 or thereabouts. Actual muzzle velocities are nice, but differences in come-ups are good to know as well.

FGM 168/175 are the rounds I'm most interested in right now. I know that different powders act differently and that different rifles can produce different results.

Thanks in advance.

 

[Vio515]

I have gotten some of this data myself and some has been researched. There is a site, long range hunter, I think, that had a pretty long list of temperature to velocity shifts. I cant seem to find it currently but I know its there somewhere. Hope this helps.

Alliant PowerPro 2000mr
.6-.75 ftps per 1 degree temp change

Alliant Reloader 15
1.2 fps per degree?
1 fps / degree from 60-90 .... but from 10 to 30 it may be 2 or 3 fps / degree

IMR 4064
.53 fps per *
Varget
.55 to .72 fps per degree?

 

[CavReconScout]

I have gotten these results from my Savage 10TR 20" 5R using Federal 175 GMM with a Pro Chrono Digital @ 15':

77* 850DA 2632fps
82* 2500DA 2597fps
66* 1100DA 2600fps

Just FYI I also chrono'd some Nosler 168 CC HPBT using the same info as above:

82* 2500DA 2507fps

 

[colson]

Thanks for the info guys, keep it coming. I'm trying to nail down this temp vs. muzzle velocity thing for FGM before I head back to the range this winter. The last time I shot was in the spring. I'm really interested to see how this ammo performs over the whole temp range.

 

[Luky]

24" Lilja 1:11 barrel in LR-308. 73F 29.36 in Hg
Prochrono digital @ 15' (note the MV data for prochrono appears to be aboubt 34fps lower than data taken with a magnetospeed for my pet varget load (prochrono vs magnetospeed) on same rifle. Did not test FGMM with Magnetospeed).

15 rnds 168 FGMM

Avg (fps) 2702
Hi 2715
Lo 2682
ES 33
SD 9.87


Guess I should also note the box of FGMM was bought about 1999. Note sure if that would make a difference vs current FGMM? (why my MV is so much higher than what you show).

 

[colson]

2700 seems smokin hot for 168 FGM. I thought my 2575 number was low, but it took 17 MOA to hit at 540 meters and 21 MOA to hit at 620 meters, so that mv input works in JBM.

 

[ShootsBlanks]

My 24" Savage produced an average of 2698 fps last February with 168 FGMM that was current production at the time. The gun loves the stuff.

Recently, I became aware of multiple local PDs running into bullets being pulled from the case if it was extracted rather than fired. They all have current production ammo. We compared the base to ogive measurement and found current production to be 0.0202 base to ogive longer than before. This is sample of only five rounds from each of four lots. Two old lots and two new lots. The OAL length is consistent. I understand that Sierra has confirmed a change in the 168 SMK bullet profile.

The difference is even greater with some old Blue and White box FGMM.

 

[colson]

It would seem that my rifle is a bit slower than most, at least with the ammo that I was using at the time. I hope to get out in the next few weeks and see whats what.

As I understand it different chronos will produce different results. Ultimately, I'm not going to get wrapped up around the axle about the true MV; I just want an idea of how much variation I can expect across the entire range of possible temps. That way I'll have a chance to be on target when I go to verify at those temps.

Thanks for the input, all.

 

[CavReconScout]

I went out today and re-zeroed my scope. Conditions were 32.5*, -713 DA, freezing rain, little or no wind. Savage 10TR 20" 5R, Pro Chrono Digital @15':

2604 fps, 55ES, 24SD

Even tho conditions were less than ideal, prone with bipod and rear bag I still managed to shoot a .80" 5 shot group at 100yds. As far as come ups, I use a FDAC set @ 2600fps, and the data has been accurate for me out to 800yds.

 

[colson]

crs: Great info! It seems that you are not seeing a significant shift in MV due to temp???

 

[CavReconScout]

I am amazed at how consistent the FGMM 175gr has been. My reloads using 4064 have also been amazingly consistent at about 25fps slower over the same 50* temperature difference. I get slightly tighter groups using Re15, but the velocity swings at different temps are significantly greater.

 

[HD1911]

2700 seems smokin hot for 168 FGM. I thought my 2575 number was low, but it took 17 MOA to hit at 540 meters and 21 MOA to hit at 620 meters, so that mv input works in JBM.

I was getting 2,750 FPS with FGMM 7.62 175gr out of a 24" Bartlein...

 

[colson]

I was getting 2,750 FPS with FGMM 7.62 175gr out of a 24" Bartlein...

What temperature?

 

[HD1911]

What temperature?

40 - 65 Degrees F.

All FGMM I have ever shot, has been Fast. But especially the few different lots of 7.62 175gr that I was using.

 

[colson]

40 - 65 Degrees F.

All FGMM I have ever shot, has been Fast. But especially the few different lots of 7.62 175gr that I was using.

Do you find that the chrono velocity matches up with the required come up from your ballistic computer? I thought that the chrono I was using was reading slow, but the MV input was giving me dead on elevation come ups, so I'm thinking it was good.

I'm learning that Federal uses different powders for different lots, or they have in the past. I'm wondering if the temp variations are consistent with the different powders. I'm probably a little OCD about this, but without sufficient range time the best I can do is ask others what they are observing. Hope to shoot tomorrow out to 700 meters, so I may gain a bit of new knowledge.

 

[HD1911]

yeah took it out to 1,050 Yards and using that input velocity of 2,750 worked for me

my Bartlein may have been a Tightbore, not really sure. maybe it was faster than most other barrels? either way, even out of an old Savage 24" tube, I was getting roughly 2800 with 168gr FGMM

 

[Knebraska]

I'm new at this sport, but this morning I shot before work. The outside temp was 2 degrees. By the time I arrive at my shooting spot, the .308 rounds i hand loaded with 42gr of IMR 4064, in Lapua brass with win primers, all set at 2.80col, that were stuck under my windshield wiper blades until I arrived at the shooting spot, laser temp reading was -6 degrees.

I shot off hand through my chrony. I fired four rounds and the average speed was 2513 fps. A few days previous, I fired four rounds when the outside temp was 42 degrees and got an average speed of 2617fps. Thats 104fps difference with 48 degrees of temperature drop.

I have read in most places that IMR 4064 is not temp sensitive.

Granted, I 'm new at this, but I hand loaded and weighed on a scale each charge. I shot these through a Rem 700, 20" bull barrel 1/10 twist with a YHM suppressor.

Can anyone confirm any similar variance with the SUPER cold shots?

thanks