NZ Longranger
Well-Known Member
JB - I'm packing for the trip but this is too interesting to walk away from just now...the 7mm/300 Norma imp is one of our favourite cartridges. We call it the 7mm FatMax. All our wildcats based on the Lapua head diameter start with "Fat" due to the larger head diameter over the 404 based WSM and Ultramag variants. The Max comes from its the max capacity we can fit in a short Rem 700 type action (significantly modified of course with a longer mag box etc) The 7mm FatMax is one of our most popular custom lightweight long range hunting rifle builds, and we have built lots of them over the years. (started off with the 338 Norma case before the 300 Norma came out) US 869 used to be our go to powder for this case, and the only way to keep it shooting consistently was with regular throat cleaning with bore polishes (JBs for years, but we use KG2 now). The throat surface would grow in thickness (decrease bore diameter) and roughen and stop shooting approx about the 50 round from new mark, and no amount of cleaning with the best carbon solvents would fix it. Pressure would increase, and accuracy would go to the pack. You could feel it on a tight patch and a bore slug showed it up real easily. We took barrels off believing they were shot out, but I just keep thinking on this. We sectioned and examined the barrels under a microscope, and scratched our heads some more. Then we started to use an abrasive, short stroking just the area ahead of the throat, and hey presto, shot out barrels came to life again and went back to what they had been initially accuracy and pressure/velocity wise - but needed this treatment every 10 rounds approx. I still have one of these original barrels now that looks just terrible after 700 odd rounds but so long as I keep KG2-ing the area just ahead of the throat, the dang barrel just won't die and still holds half MOA.
Then Reloder 33 came along for these sort of expansion ratio cartridges and that seriously diminished this carbon ring/throat thickening issue - but it does not eliminate it. In the 7mm FatMax its about 40fps behind 869 in velocity for pressure, but it still easily hits the accuracy node of 3200fps with the 180gn Berger Hybrid out of a 26" barrel. I had a customers 7mm FatMax come back just the other day, that had only been cleaned with solvents, and the accuracy and consistency had gone to the pack. The throat looked terrible through the borescope. I polished the throat area with KG2, and bang, .3" 3 shot groups right back where they should have been velocity wise with the same ammo. This is with a lightweight Ti actioned hunting rifle weighing just over 7 pounds scoped.
I could go one quoting cases endlessly of fixing pressure/accuracy issues with overbore cartridges buy KG2-ing the few inches ahead of the throat, but as you mentioned the 7mm/300NM, and it really was the one we cracked this issue with, I'll stop here and go back to packing!
One thing I better mention though, is our KG2 cleaning system. Now I can't claim any credit for this, as I read about it years and years ago in Precision Shooting mag from an old guy who swore by this system and who could really shoot and won a lot of benchrest in those days - I can't even remember his name. First we do our standard carbon clean as usual (we use BoreTech C4 on a borebrush, but there's several superb carbon solvents these days). Flush the bore clean however you usually do it, (we use Loctite pump bottle degreaser on patches). If you can see any copper streaks left on the bore after your carbon clean, then your barrel is not broken in properly and you will struggle to get consistent accuracy, but that's a topic for another post...we are talking about carbon ring/barrel surface structure issues here)
For the KG2, you wrap a longish patch diagonally round an undersize bore brush (we use 243 for 7mm for example), until you get a good tight fit with a lot of surface area that you can only just reverse in the bore. It wants to be tight, so the bristles spring out and push the patch into all the internal corners of the lands. Use a good fitting bore guide to ensure your rod is straight and centralised in the bore, apply KG2 to the patch, then push it carefully through the throat and about 4 inches down the bore, then reverse it back just into the chamber again. Scrub this backwards and forwards for as many cycles as it takes to remove the constriction in the bore. If you do it regularly on a bad carbon ring combination powder/cartridge (like US869 in an overbore 7mm), you'll only need 5 cycles each time you clean. If you are trying to recover a barrel that has gone west, then you might need a lot more cycles. (You can tell by testing afterwards by feel with a tight patch on a wrap around style jag. You will feel the tight spot disappear). We always finish off by pushing it right down the bore and out after the last cycle (no scrubbing, just one pass through at the end when the abrasive has largely done its job and broken down on the patch anyway), and have never been able to measure any enlargement of the rest of the bore from this at all. Do not scrub the whole bore unless you have another issue like rust, wall to wall copper etc.
You mention different barrel types, manufacturers etc and you are 100% right. This tight bore ahead of the throat issue is worst in button rifled barrels that have been fluted, as they have a bad habit of stress relieving when fluted, and opening up in bore diameter under the fluted area. If you then create an even tighter spot ahead of this with a carbon ring, surface structure change - just ahead of the throat but before the looser area under the flutes, then you can kiss accuracy good bye for sure. Squashing your bullet then having it flop down the rest of the bore to then only tighten up again for the last few inches when the fluting stops is a disaster.
Time to stop before I wear out the keys on the laptop!
Then Reloder 33 came along for these sort of expansion ratio cartridges and that seriously diminished this carbon ring/throat thickening issue - but it does not eliminate it. In the 7mm FatMax its about 40fps behind 869 in velocity for pressure, but it still easily hits the accuracy node of 3200fps with the 180gn Berger Hybrid out of a 26" barrel. I had a customers 7mm FatMax come back just the other day, that had only been cleaned with solvents, and the accuracy and consistency had gone to the pack. The throat looked terrible through the borescope. I polished the throat area with KG2, and bang, .3" 3 shot groups right back where they should have been velocity wise with the same ammo. This is with a lightweight Ti actioned hunting rifle weighing just over 7 pounds scoped.
I could go one quoting cases endlessly of fixing pressure/accuracy issues with overbore cartridges buy KG2-ing the few inches ahead of the throat, but as you mentioned the 7mm/300NM, and it really was the one we cracked this issue with, I'll stop here and go back to packing!
One thing I better mention though, is our KG2 cleaning system. Now I can't claim any credit for this, as I read about it years and years ago in Precision Shooting mag from an old guy who swore by this system and who could really shoot and won a lot of benchrest in those days - I can't even remember his name. First we do our standard carbon clean as usual (we use BoreTech C4 on a borebrush, but there's several superb carbon solvents these days). Flush the bore clean however you usually do it, (we use Loctite pump bottle degreaser on patches). If you can see any copper streaks left on the bore after your carbon clean, then your barrel is not broken in properly and you will struggle to get consistent accuracy, but that's a topic for another post...we are talking about carbon ring/barrel surface structure issues here)
For the KG2, you wrap a longish patch diagonally round an undersize bore brush (we use 243 for 7mm for example), until you get a good tight fit with a lot of surface area that you can only just reverse in the bore. It wants to be tight, so the bristles spring out and push the patch into all the internal corners of the lands. Use a good fitting bore guide to ensure your rod is straight and centralised in the bore, apply KG2 to the patch, then push it carefully through the throat and about 4 inches down the bore, then reverse it back just into the chamber again. Scrub this backwards and forwards for as many cycles as it takes to remove the constriction in the bore. If you do it regularly on a bad carbon ring combination powder/cartridge (like US869 in an overbore 7mm), you'll only need 5 cycles each time you clean. If you are trying to recover a barrel that has gone west, then you might need a lot more cycles. (You can tell by testing afterwards by feel with a tight patch on a wrap around style jag. You will feel the tight spot disappear). We always finish off by pushing it right down the bore and out after the last cycle (no scrubbing, just one pass through at the end when the abrasive has largely done its job and broken down on the patch anyway), and have never been able to measure any enlargement of the rest of the bore from this at all. Do not scrub the whole bore unless you have another issue like rust, wall to wall copper etc.
You mention different barrel types, manufacturers etc and you are 100% right. This tight bore ahead of the throat issue is worst in button rifled barrels that have been fluted, as they have a bad habit of stress relieving when fluted, and opening up in bore diameter under the fluted area. If you then create an even tighter spot ahead of this with a carbon ring, surface structure change - just ahead of the throat but before the looser area under the flutes, then you can kiss accuracy good bye for sure. Squashing your bullet then having it flop down the rest of the bore to then only tighten up again for the last few inches when the fluting stops is a disaster.
Time to stop before I wear out the keys on the laptop!
