I know the current thinking is to build “Across the Course” rifles on an AR frame (space gun) or something along the lines of a Tubb rifle but I have always enjoyed shooting a bolt gun. I am currently in the process of wringing out a new rifle I chambered in 6mm/223.
I have watched a change in thinking for NRA High Power across the course in match rifles and calibers. There are several newcomers to the field; the 6mm AR, 6.5 Grendel, 6XC, and 6.5 Creedmoor. I have seen these rifles perform and all have done very well.
When I started this project I was looking for a cartridge that shoots and feeds well from a magazine or clip. I had shot the 223 Remington in my 722 Remington and knew it had fed well so I chose the 223 Remington necked up to 6mm.
The 6mm/223 gives me what I have in the standard 223 only with a better selection of bullets. I wanted a better 600 yard bullet with a higher B.C. than provided by the 80 and 82 grain bullets. I also wanted a 600 yard line bullet with a .500 or better B.C. for those shots that should be 10 ring or better but seem to find the 9 ring no matter how well I held and broke the shot.
My project started with ordering a 6mm barrel from Bartlein with a 1-8" twist. I called Pacific Tool and ordered the 6mm/223 reamer. I found a used Remington 700 action at a local gun shop and already had a Remington 40X stock. I then started putting the gun together.
I called Jay at Redding and talked to him about the die set that would work best for this project. I ordered a Type “S” Match Full Length die set for the 223 Remington and long and short 6mm seating stems. I reamed a competition seater sleeve to accept the 6mm stems and then ran my chamber reamer inside the sleeve to cut the neck for the 6mm/223. I used the full length die with a .264” bushing and checked to make sure this had enough clearance for the case neck and die.
Next, I went to my good friend Neil Keller at “Kustom Ballistics’’ for some instructions on “blue printing” the action and chambering the barrel. We used Neil’s lathe for these operations. I had also purchased the following tools for these operations from Brownells: a Bolt lapping tool, Receiver facing mandrel, and a Receiver Accurizing System.
After making sure everything was straight and true we threaded, chambered and headspaced the barrel. Finally, we cut the barrel down to 27” and finished the muzzle with an 11 degree target crown.
After the metal work and stock work were done, I turned my attention to working up some loads.
I used Berger 70 grain and Sierra 70 MatchKings for my 200 and 300 yard loads along with Vihtavouri N540, Remington cases and Federal 205M primers. The initial load gave me .200” -.250” groups off of the bench at 100 yards. Thank God, not much load work up here. These loads were derived from the Sierra load manual using the 223 Remington Bolt-Action Data
The 105 grain load was a little more difficult. The accuracy was about the same but the extreme spread was too high, around 30-40 fps. I was looking for 20 fps or less, thus am still working on the 105 grain load.
Left to right: 6mm/223 – Berger 95 gr VLD and Berger 70 gr, 223 Rem. - Sierra 69 gr. and Sierra 82 gr.
I tried 95 grain Berger VLD’s with Vihtavouri N550 and RL15, both of these powders produced very good results. RL15 load has an E.S of 4 FPS and Vit N550 had an E.S. of 9 fps. No load data was available from published books so I worked the loads up from water volume and Quick Load.
Preliminary testing at 300 yards produced very good results. My first 600 yard match gave me the feedback I wanted. You only get 2 sighters in a match and for a new rifle with no 600 yard sight dope I was a little concerned about being on paper with the first shot. Fortunately, I grabbed paper but took 6 more shots to get zeroed. After losing some points for the zeroing effort, I shot three 9’s on the last 15 shots for record and the rest were 10’s and X’s. My score for that match ended up being a 186-5X.
Target shot at 100 yards in prone position with iron sights. 22 shots with Berger 70 gr. Bullets in 6mm/223 Rem.
I have more work to do on this project but so far this cartridge looks promising for what I wanted to accomplish. I will keep you updated on my progress.
The 6mm/223 is an old chambering but shouldn’t be overlooked if you are shooting an AR-15 configured rifle or want a bolt-action that will feed well in the rapid fire stages (National Match courses).
Sinclair Tech Staff
NRA Metallic Reloading Instructor