Originally Posted by Kevin Thomas
Not to dive into the hornet's nest here, but yes, a 1x12" would worked just fine for what he's looking for here; the 168s and 175s will work in the 1x12", or even 1x14s for that matter. I've shot quite literally millions of both of these bullets in both twists.
The 168s, at least the SMKs are not, and never have been 1,000 yard bullets. They were designed specifically for 300 meter competition, and have been used (by some) for the 600 yard line as well. The vast majority of us shooting bolt guns, however, shot the 168s at 200 and 300, and switched to the 190s or even 200s for the 600 and 1,000 yard lines. In the M14/M1As, the 168 was often used at 600, but for the 1,000 yard line, the 180 was the go-to bullet (can't use anything heavier than this out of consideration for the gas system in the M1 and M14), as the 168 would go subsonic at around the 900 yard mark. Asa is off just a bit in his comments about the 180s; the redesign was done in the mid 1990s. It was a stupid thing to do, and there were one or two of us (their only competitive shooters) who railed against this switch, which involved changing the boat tail angle and length. The intent here was to allow the use of the same BT punches and form dies as the 168, thereby simplifying things for the machine shop. It also dropped the BC significantly, and made the bullet completely useless for what had been its main reason for its existance. We were over ridden, and the change was made. They had to continue to make the old style, long boat tail design for AMU on special orders after that, because they (AMU) wouldn't use the new design. The M16s came on the scene a few years after that, and made the whole issue moot anyway. Don't see many .30 cals on the line anymore, in almost any flavor. A foot note to this is the development of the 175 currently used in the M118LR load. This bullet was developed when Lake City needed an accurate bullet for long range sniper ammo, and to replace the M852 168 grain Match loading. The reason they just didn't use the M852 for this application, was that it just didn't perform at 1,000 yards. The 175 was designed with some of the same features as the old 180, and will stay supersonic out to the 1,000 yard line, no problem. Again, the 175 was the upper weight limit allowed as the ammunition could have seen use in the M24s or M40s (bolt guns) or the gas operated M21s.
There was a question as to why the 155s would work (stay supersonic at 1,000) and the 168s wouldn't. The 155 is a bit of a special case, and worth some explanation. To begin, none of us use the 155s beacuse they're the "best" bullet for 1,000 yard shooting. They aren't, not by a long shot. We use them because they're the max weight allowed by international Palma rules, period. My own Palma gun is typical, in that it uses a 1x13" twist and a tight bore (.2980" bore, .3065" groove, if I recall), being set up specifically for the 155s. In this application, there were several "violations" to bullet design that were incorporated to result in a bullet that would stay supersonic when fired from a 308 Win case. It has a very short bearing surface (less that the normal one caliber required for most designs), a fairly long boat tail and a sharper ogive (the combination that results in reduced bearing surface), and an extra small meplat. Even with all this, the bullet still needs at least 2,950 fps to be comfortably supersonic at the 1,000 yard line, which is why virtually all Palma guns run 29"-30" barrels. Take the same gun and load, but substitute the normal 150 grain SMK, and you've got a tumbling, subsonic bullet long before it reaches the target. Given our 'druthers, most of us would go for a 190-210 grain bullet in a heartbeat. In fact, most US Palma competitors who don't have any intention of competing in International matches, or those within the US that require the use of the 155s (such as the US Palma tryouts, and some other matches), set their guns up for, and use these heavier bullets. It's a matter of match requirements, not superior ballistics, that brought the 155s to the fore today.
As I said, not trying to stir the hornet's nest any more, but there seemed to be a few things that could stand some clarification. Hope this helps.