Thinking about a 6.5 cm semi-auto but have concerns about how reliable it will be

[houndog]

I was originally going to post this in the last 6.5 cm thread, but didn't want to hijack that thread. I have a 6.5 cm bolt gun, and have now been seriously thinking of adding an AR10 in this caliber. In particular, Larue has a build your own deal going right now. I've ordered a .308 version, but have been struggling whether I should change to 6.5cm. On the one hand, 6.5cm has better ballistics, particularly at distances in excess of 600 yards. Also, I think the factory match ammo is a little better than factory match in .308. Finally, I do already have an AR10 in .308 although it's not really set up for long range shooting.

On the other hand, right now all of my shooting is now under 600 yards (no access to a range with longer distances until I move to CO next year). Also, I can get good quality match .308 for .80-1.00 per round, and I have a ton of cheaper plinking ammo, so it would definitely be cheaper to shoot the .308. Then there is the fact that .308 barrel will probably last twice as long. Finally, I do have a concern about the reliability of 6.5cm in a semi-auto. Those of you that have the round, what is your experience? Does the round reliably feed and extract from your gun? Is your gun mag sensitive?

 

[Roslyn]

Would offer that you need to ask yourself what your purpose is here in order to answer the question. You seem torn between the tradeoffs around cost, durability (which is cost in another dimension) and precision. Would suggest you land on either cost or precision and make a call.

A well built AR10 in 6.5 CM will run as reliably as a .308; won't be any more mag sensitive than a .308; and will give you 2500 rounds more or less of barrel life. It will also be a hell of a lot easier to shoot well, meaning accurately, unless your fundamentals behind a .308 are rock solid. So no downside to a 6.5 CM as a platform, in and of itself.

If cost is a major concern, and you intend to keep shooting inside 600 yards, you may be better off with an AR15. If you expect to shoot out past 600 yards, a 6.5 CM is so superior to a .308 ballistically that it isn't a contest. Maybe you prefer the challenge of shooting a .308 well at distance. Plan on having at that for a long time, because a .308 is very demanding on your fundamentals. A 6.5 CM will cost more for factory ammunition. The good news is that factory Hornady, or Prime or Eagle Eye are fantastic, .5 MOA zip code rounds. The Eagle Eye .308 rounds are also exceptional, but don't price out at $1.00 a piece. Not sure what you can find in .308 ammunition that is $1.00 a round and shoots .5 MOA.

My personal net on those tradeoffs is that precision shooting at distance is very difficult and the advantage of a 6.5 CM makes it a better choice. If cost is more important to you, the 6.5 CM with commercial ammunition won't win the day.

I don't own a LaRue so haven't anything to contribute there. Two things I would focus on, whatever you do, is to get a first rate trigger; and an adjustable gas block. Those alone will make a world of difference whatever you decide.

 

[houndog]

Thank you for your thoughtful comments. You are right, I really need to decide exactly how I'm going to use the rifle and how important superior ballistics are at the distances I will be shooting. Happy to hear that 6.5cm seems to be a reliable platform. That was one of my big concerns.

Also appreciate your suggestions on factory ammo. I can get .308 FGMM shipped for $1/round. Also, I have a bunch of it already. Doubt it's capable of regularly shooting sub .5 MOA, although sub 1 MOA is realistic if I do my part. Good to hear that some of the 6.5cm factory ammo is so accurate.

Realistically, barrel life is probably the least of my concerns. Given the cost of ammo, optics, etc. having to replace the barrel after 2500-3000 rounds shouldn't really effect my decision.

Curious why you feel that the 6.5 cm is so much easier to shoot at distance. I assume it's just the ballistics - i.e. bullet drop and effect of wind is much less on the 6.5cm, which is certainly an important factor, especially for someone like me who is relatively new to long distance shooting.

I agree about the trigger. The Larue's come with a MBT trigger, which I have in an AR and like. Pretty comparable to the Geissele (sp?) I have in my other AR. Why an adjustable gas block though?

 

[Roslyn]

The 6.5 CM ballistics mean your elevations and windage adjustments will be more advantaged relative to a .308 175 gr round, as one example. Run the JBM (or frankly any other point mass solver) ballistics program on a representative 6.5 CM round and a .308 round, it will be obvious. Use a 9 o'clock, 10 mph wind value for both calibers. My program shows a 175 SMK running at 2700 fps muzzle velocity needing 10.2 mils of elevation and 2.6 mils of windage at 1000 yards. A 6.5 CM 140 gr Hornady ELD-M running at 2700 needs 8.9 mils of elevation and 2.0 mils of windage. The Prime 130 gr round might need only 7.3 mils of elevation and 1.8 mils of windage. A mil at that distance is 36 inches, so it adds up.

The adjustable gas block will let you tune the gas pressure to the factory ammunition you end up shooting. Absent that tuning, it will be accidental to get the rifle to run correctly -- reliable extraction, ejection and so on -- with factory loads.

 

[houndog]

It's that last sentence that worries me. Larue's will not come with an adjustable gas block, although I suppose you could have an aftermarket one installed. It's just that I have an AR10 in .308 and several AR15's none of which have adjustable gas blocks and all of which run fine with whatever ammo I put through them. If AR10 in 6.5 CM need an adjustable gas block and must be constantly reset each time you use different ammo, that would be a concern.

 

[padom]

You dont NEED an adjustable gas block. You SHOULD HAVE an adjustable gas block to get it running optimally. And no, I havent had to reset my AGB each time I put different ammo through it. I have now had 3 6.5cm semis and all I did was run 140 Amax or 140 ELD-M through the first two and set gas to lock back on empty mag. Never had to touch it again. My most recent 6.5cm semi by PVA with +2 gas is the smoothest shooting large frame semi Ive owned. With the JP SCS, SLR AGB and 22" PVA Bartlein with JP Barrel extension and +2 gas this thing doesnt put a swipe, scratch, dent, etc on the brass and is extremely accurate. Im shooting 123 Lapua Scenars over 43g H4350 in Hornady brass and 210M primers.