What would cause a 350 fps loss in velocity?

[jimhoodag]

My 6.5 x 284 Shilen match, select 26 inch barrel has lost 350 fps (2,950 to 2,600 fps) with the same load. It has about 500 rounds down it. Although I clean the gun every 25 rounds or so, I am thinking it is copper fouling, throat erosion, my new sonic cleaner is leaving moisture in the casings or the barrel is shot out.

I shoot Berger 140s touching the lands. When I cleaned the copper down to the metal and scrubbed the throat, I noticed the throat had eroded .020 since new. This jump may have caused a decrease in pressure, but I can't see it causing a 350 fps drop.

My chronograph works because my other guns clock at correct speeds. If barrel were damaged or shot out, I thought that it would just become inaccurate. Mine still shoots 1/2 moa. The gun safe is in a heated and cooled room, so I don't think barrel would be pitted.

I am getting ready for deer season and need help. What would cause such a drastic 350 fps drop in velocity?

Jim Hood

 

[rduckwor]

Same lot of powder before and after the loss?

RMD

 

[Bryan Litz Ballistics]

350 fps is huge. I suspect one of two things:

A) Chrono wasn't assembled correctly when you measured these numbers. It might be accurate on a different set up, for a different gun... but maybe just put together wrong in the one session you measured -350 fps. Would need more details on the chrono to further diagnose this possibility.

B) Powder scale set up. Again, need more info to be sure, but is it possible you set up the powder scale with like 10 grains less powder? Possible mistake with balance scale, less likely with electronics.

I don't think something like fouling, pitting, or powder lot change would cause an actual shift in MV of 350 fps. Different type of powder maybe...

-Bryan

 

[XTR]

If it's a Alpha or Beta Chrony I've seen huge swings caused by the angle of the sun, especially with clear skys, or the sun coming out from or going behind a cloud. I'm in the market for something new, but in the mean time I've learned to do all my chrono work on overcast days.

 

[dkhunt14]

Years ago I had a lot or R22 that raised the velocity over 400 FPS. Now this was with a 240 Sierra so the lot was extremely hot. It ruined a lot of new cases even when reducing by more then 5 grains. Matt

 

[jlow]

When you hear of something like this, the chrony is always suspect and time after time it comes back to the chrony. I agree that even if the chrony worked recently does not mean anything as long as the other test rounds were not shot in the same session.

 

[jimhoodag]

Thank you all. Same lots of RL17 and H4831sc and same charge (about 49.5 of each) thru a RCBS electronic dispenser and electronic scale. I shot other rifles in cooling rotation thru chronograph in same position on a cloudy day.

It shot 2,600 fps in early July, so I cleaned the copper and powder down to bare metal. I shot about 15 rounds in mid-July and it went back up to 2,930. Chrono was in shade on clear hot days. I cleaned powder, but left copper thinking it needed a little copper. When I shot it last week, it was back down at 2,600.

I forgot to mention that did my first neck turning experiment on some of the Lapua brass and full resized it. I necked sized the others with a Redding bushing leaving about one to two thousands of neck tension. The fired cases were not smoked or unusual, so I doubt it was a neck tension issue. It still could be moisture in powder from sonic cleaning.

I think you guys may have something on the chronograph. This 6.5 has a Nightforce 56 mm bell and a 20 mm mount, which may have sat lower on the front rest causing the bullet to pass lower/nearer to the chronograph. The other scopes were 40 and 50mm. Would this cause a 350 fps drop?

I only have a 350 yard range at my farm, which might not be long enough for me to tell a 350 fps loss. There should be a two inch drop difference between 2,600 and 2,950 in a 140 Berger VLD, but I don't remember noticing it at 300 yards. It grouped at half moa near the center, but I don't remember whether I used the 2,600 or 2,950 drop calculation. I'll clean the copper out and set the chrono just right for this gun and see if there is a two inch drop.

I also bedded a new stock on this rifle too close to deer season - too many variables for me.

Thanks again.

Jim