6.5 PRC MV SD Issues

[shootingnut]

Got a Bartlein 7.75tw 26" SS barrel on an Impact 737r action and been having some weird velocity issues. It has about 200 rounds on it and was showing great low SDs with some Copper Creek loads and Norma Golden Target factory ammo, in the 5-8 SD ranges. Then all of a sudden it started showing that same ammo with SDs of 15-20+. I figured it needed a cleaning, so I used Boretech C4 and gave it 3 good cleanings leaving the barrel soaking with the compound in-between each cleaning in case there was a carbon ring.

Now after cleaning, the same ammo that was good is still showing 12-17 SDs. Could it have anything to do with temperature affecting the Magnetospeed? Was shooting in 20F weather with some wind chill, whereas it was more in the 50s the last time I checked. The velocity numbers were slower, which was expected given the weather. But I haven't heard of cold affecting MV SDs or accurate chrono reading.

Or perhaps the barrel still needs more cleaning? I suppose there's potential that a carbon ring is still there, but I don't have a borescope yet. Or maybe it's the opposite and the barrel needs to foul up again? It probably had about 15 rounds through it once I started testing the "good" ammo, so I figured that would be fouled enough.

Appreciate any ideas you guys would have. Thanks!

 

[spife7980]

Could be the weather, could be the fouling, could be the weather in combo with the fouling, could be thats what it was all along.

 

[diggler1833]

By "same ammo", does that mean the same lot too?

 

[shootingnut]

By "same ammo", does that mean the same lot too?

Yep, same lot of each respective type.

 

[TylerN]

How many rounds were shot to measure those SDs?

 

[shootingnut]

How many rounds were shot to measure those SDs?

10 shot strings, also giving time for the barrel to cool between strings.

 

[diggler1833]

Getting after the carbon ring is a whole separate animal from cleaning the bore of the barrel. I purchased a Teslong borescope that simply attaches to your laptop or phone for under $100, and it has been an excellent tool.

I cleaned my 7 mag down to bare steel after this last season. It had ~220 rounds total on it, and about 75 since the last decent cleaning. I scoped it, and to my surprise...a pretty good carbon ring had developed already. 20 minutes of spinning a .308 nylon brush by hand, and it was mostly taken care of.

I'm not saying that yours is a carbon ring issue. However it sounds like you have enough repeatability in your other steps to wonder what's going on. Perhaps it is a temperature/ignition issue too.

Any change in accuracy?

 

[shootingnut]

Getting after the carbon ring is a whole separate animal from cleaning the bore of the barrel. I purchased a Teslong borescope that simply attaches to your laptop or phone for under $100, and it has been an excellent tool.

I cleaned my 7 mag down to bare steel after this last season. It had ~220 rounds total on it, and about 75 since the last decent cleaning. I scoped it, and to my surprise...a pretty good carbon ring had developed already. 20 minutes of spinning a .308 nylon brush by hand, and it was mostly taken care of.

I'm not saying that yours is a carbon ring issue. However it sounds like you have enough repeatability in your other steps to wonder what's going on. Perhaps it is a temperature/ignition issue too.

Any change in accuracy?

Hmm, I don't have a borescope but I decided to stick a sopping wet bore mop soaked in C4 into the chamber and let it sit for a couple hours, refreshing the mop each time, and then span a nylon brush by hand like you described. I'll take it out again this weekend and report back. As far as accuracy, it was great, just the MV SDs that were bad.

 

[shootingnut]

Getting after the carbon ring is a whole separate animal from cleaning the bore of the barrel. I purchased a Teslong borescope that simply attaches to your laptop or phone for under $100, and it has been an excellent tool.

I cleaned my 7 mag down to bare steel after this last season. It had ~220 rounds total on it, and about 75 since the last decent cleaning. I scoped it, and to my surprise...a pretty good carbon ring had developed already. 20 minutes of spinning a .308 nylon brush by hand, and it was mostly taken care of.

I'm not saying that yours is a carbon ring issue. However it sounds like you have enough repeatability in your other steps to wonder what's going on. Perhaps it is a temperature/ignition issue too.

Any change in accuracy?

That did the trick, after leaving that bore mop soaked in C4 in the throat, and then using a nylon brush and spinning it by hand in place, took the barrel out yesterday and the SDs were right back where they used to be. I didn't get a borescope but I'm thinking it was a carbon ring.