Greetings,
A few days ago DaveI and I were discussing the 7mmBR / TPP cartridge for HP Hunter Class. We digressed to Dave’s 7mm TCU, which he shot for years back in the late 80’s and early 90’s. I believe that timeframe is correct, if not Dave can correct as needed. Anyway, Dave shot that “dinky” cartridge launching the 168 Sierra MK’s at rams and the 130 Sierra MK’s at CPT’s.
Bullet selection was not very good back then from a high BC perspective. The Sierra 168 MK has a BC of about 0.485, a bit on the low side for trying to nail rams in windy conditions. The 130 Sierra MK could reasonably be called atrocious from a BC perspective as that low of a BC has a most deleterious affect on oneÂ’s attempt to hit turkeys due to wind deflection issues. The BC of the 130 Sierra MK is around 0.380 if my memory serves me well. We are talking about a tough time at turkeys if there is any wind blowing about on the range. At 7mm TCU MVÂ’s the 175 Sierra MK has a 0.600 BC while the 139 SST Hornady has a BC in the neighborhood of about 0.495 when launched at 2,400 fps. One could also consider shooting the 175 Sierra MKÂ’s at about 1,800 fps for the short line. That would give better wind performance than the 139 SSTÂ’s and very mild recoil of about 6.5 ft-lbs. A typical 260 launching the 107 MK to 2,700 fps has a recoil of about 7.1 ft-lbs and the slow TCU round would not leave any pigs standing when hit.
Now, however, with the improved selection of high BC bullets from Sierra and Hornady the 7mm TCU is sounding very interesting when an 8-twist barrel is used.
My SWAG at projected MVÂ’s for the very nice 175 Sierra MKÂ’s is about 2,225 to 2,250 fps using a full case of VarGet or N140. That will produce a terminal momentum at the ram line of about 1.22 ft-lbs, well above most 260 ram loads and my, my, my, would the recoil be mild. We are talking about a ram-load recoil of about 9 ft-lbs compared to the average 260 load of about 12 ft-lbs out of 9 lb rifles. Out of a Standard rifle at full weight the recoil would be about 8 ft-lbs. Of course one could down load the 175 MK ram load for even more reduced recoil. IÂ’d say that even at 2,000 fps the 175 MK would still have a very high ram knockdown rate.
For CPT the very fine 139 Hornady SST would be launchable to about 2,400 fps, plenty for fine work on the short line.
Given that there is now factory brass for the 7mm TCU and that it has been chambered in rifles for years, T/C rifles, it would seem to be an ideal HP Hunter cartridge for the recoil shy shooters out there. For those girls and boys of yours that are interested in HP Hunter or Standard this just might be the cartridge for them and they are not likely to ring rams either, a frustrating experience for oldsters and youngsters alike.
One caveat though, most 7mm TCU reamers out there were designed for shorter bullets and will not work for the above listed 175 SM bullet, which really make the 7mm TCU a viable contender for top match performance. A properly designed reamer would need to be ground for HP Silhouette rifle application.
