Mitch, 600 is like any other BR match, you can be as competitive as you want to be or what the finances, or the minister for finances will allow
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600 is very popular with our group here in the North because the target is a lot easier to use when the mirage is up, but moreso because there are two clear sections of the competiton. Score and group. You can scew one up and do good in another. Of course you can screw them both up on a horror day like any other form of BR.
To answer you directly, a half minute rifle can do very well if the driver is capable. Factory rifles in the right calibers can do well also, but to be honest it is the aggregates that suffer with these sorts of rifles. 600 like 1K are about 6 and 10 match aggs and day matches are taken as part of the ladder to the end of year aggs. On our range, good day aggs of the four cards can be under or around 3 inches for group and over 200 for score where the possible is 220. The 1.75 inch X ring scores as an 11.
If you are asking as to the actual rifle requirements to do very well, my criteria with a LG is sub 2" groups at 600 yards during controlled testing, and with a HG, i would look to sub 2 inch aggs during controlled testing. BC of the bullets should be .500 or better with a 3000 fps range. Less than this is behind the eight ball on tough wind days. Actual calibers to choose can create more debate than i am willing to read about, but what does well at Fly should do well at 600, and what does well at 1K will do better from my point of view. Our range is the only range to shoot organised 600 at this time, and all calibers have been used at some time over the last couple of years. One small group means fuckall to me, one high score also means fuckall to me, the six match yearly aggs mean everything to me and they are clearly dominated by the 30 cals and the 7 mils are a close second.
One thing i will add to this, to build a rifle that will do 600 yards, 500 Fly and 1K all legally is not the way i would go. If you wish to do well at 600, build it to the rules, whatever they may be in the future, but our rules are based along IBS rules now with some added from likely ammendments to future 600 IBS rules. ie, Legal LGs are eligible to shoot in HG. Doesn't sound like much, but LGs are allowed muzzle brakes whereas HGs are not. Does it level out the class or even tip it towards the LG? IMHO, there is no substitute for another 50 pounds of a giant sloth sliding on rails. But i will say my LG, brake and all, did show up a couple of HGs this year, but in the six match agg, i am way way off the pace. Long way off. Group anyway. Score i am in front. Group is where it's at.
To win HG with a LG is not what i would want to do, i would just build a better HG like i am currently doing. This is just purely about giving people an option and the yanks like options and just love to shoot the same gun in both classes. No-one at our range has ever tried to do this, but for those silly enough to try it, the option will be there. I built my latest LG to garnish every edge i can to do well in that class and stay within the rules. It was never built to dust some HG pride. More likely my pride would get dusted.
Jethro Bodine.