Been asked for awhile to get this posted up. Here's basically a draft copy just to get folks rolling. I'll be chewing on this and reposting so check back if your interested:

The 6mm Competition Match

Purpose:

The following information is targeted for those interested in competitive tactical style shooting. This is a detail oriented presentation because it has to be. The assumption is made that the reader has experience with hybrid/wildcat type cartridges and adheres to safe reloading procedures.

It is also assumed that you are fluent with the vocabulary unique to precision reloading. If you are not, STOP. Educate yourself, then proceed. Any and all information provided is done so on a reference basis.

LongRifles, Inc. will not be held responsible for any liability. You are responsible for your own destiny, your personal safety, and the safety of anyone in your immediate surroundings when handling firearms and ammunition. Reloading can be extremely dangerous if approached with a cavalier attitude.
BE SAFE, BE SMART, and HAVE FUN!

Background:

Joe Hendricks, a respected and highly experienced NRA Highpower and Long Range competitive shooter, is credited with the development of the 6mm Competition Match. The goal was to develop an exceptionally accurate 6mm cartridge in a short action format that functions reliably with detachable magazines. The final requirements were delivering both high velocity and greater barrel service life with heavier 6mm projectiles.

PRS (Precision Rifle Series) and other tactical style matches often have courses of fire that require well over 200 rounds for a single event. The consequence of all this great shooting is barrel mileage. This is where the 6mm CM starts to distinguish itself from other popular 243 caliber cartridges. The added cartridge capacity combined with single base, magnum rated propellants covers all the bases. Barrel service life can easily exceed 3500 rounds with this cartridge.

The highest velocity we’ve attained thus far approaches 3400fps with the 105 grain Berger Hybrid target projectile. While this velocity is completely outside the rules for almost all PRS type events, the benefits in a “paper match” become immediately obvious.

The 6mm CM is a hybrid of the 243 Winchester. The case body is fire formed and the shoulder is altered to 31*. The increased case capacity (we’ve loaded this cartridge in excess of 52 grains of Retumbo) translates to impressive velocities while still preserving the original body taper. The wisdom behind this becomes immediately evident when running the bolt in a rapid fire scenario. Fluid and efficient bolt manipulation is still possible and this means the whole system works for the shooter instead of against him.

Brass selection:

This cartridge is made by traditional fireforming. Hornady does manufacture hydraulic forming dies on a custom order basis so that too is an option. On a properly prepared case, the primer pocket will ultimately determine the expiration date of the brass. Our research has shown that out of Norma, Lapua, and Winchester brass, Norma cases consistently offer the greatest service life.

Case preparation for fireforming:

A variety of methods have been used successfully to fireform cases into the 6mm CM. Packing cases with Cream of Wheat along with a mild dose of pistol powder and sealed with Crisco is a method one of our team shooter strongly advocates. I personally cannot stand the mess or the lingering odor of scorched vegetable shortening. Others tighten up the neck tension on the parent case and seat a bullet extra-long with a moderate charge of H1000.

I like to form a false shoulder on the cases because it offers the reassurance that the cartridge head is firmly seated against the bolt face when fired. This is absolutely critical with any modified shoulder wildcat cartridge because the case body must only be allowed to move in the neck/shoulder and neck/body regions. Growth in the web of the case leads to premature case failure with potentially lethal consequences.

The importance of understanding and respecting this fact cannot be overstated!

The false shoulder method’s principle advantage over other procedures is that it removes all the concerns of where the cartridge is positioned in the chamber during fireforming. When done properly, the false shoulder should create a .004” interference fit while chambering. This “crush” ensures the case cannot move forward and also delivers the assurance that the case head is firmly seated against the bolt face. This means the brass is much less likely to move during ignition as the striker impacts the primer. The only real disadvantages to this is that it involves two more steps at the reloading bench and case neck annealing should be considered if maximum accuracy and brass service life are desired.

The subject of accuracy in regards to annealing the neck may raise questions and some skepticism. What we’ve come to learn is that cold working brass (stretching/compressing the material in the absence of added heat) alters the hardness of the material. Fluctuations in hardness leads to variances in the release tension of the bullet when fired. This variance reveals itself as increased extreme spread and higher standard deviation values when evaluated with a chronograph. At distance these swings in velocity appear as vertical dispersion on the target. Annealing the case mouth/neck brings the hardness to a more uniform condition. As a result the cascading effects are reduced and accuracy improves.

Case preparation using the false shoulder method:

For the purpose of this document, I am going to skip over fundamental case preparation practices. We are only going to focus on what needs to happen to form cases from 243 Winchester to 6mm Competition Match.

I begin by using a Sinclair International Gen II Expander Die (P/N: 749-011-715WS) fitted with a +.002” 6.5mm expander mandrel (P/N: 749-001-179WS ). The case mouth is treated with Imperial Sizing Wax ( P/N: 749-001-052WS ) to reduce friction as the mandrel increases the neck caliber from 6mm to 6.5mm. Diligent use of the case lube will enlarge the case mouth and greatly mitigate the risk of turning cases into “accordions” from shoulders collapsing. In a layman’s explanation, we are turning the 243 Winchester brass into a 260 Remington.

Once enlarged, we size a portion of the neck back to the 243 diameter. I use a .270” diameter TIALN coated Redding neck bushing for this. (P/N: 749-003-502WS) For this step its ok to use either a 243 Winchester neck sizing only or a full length neck sizing die. The die should have the ability to use interchangeable neck sizing bushings. (P/N: 749-006-019WS or 749-006-023WS) The idea here is to reduce the bulk of the neck back to a typical 243 Winchester. We’re going to however leave a small portion of the 6.5mm neck in place. This little ring is going to pinch against the neck/shoulder junction in the rifle’s chamber. “The pinch” should be around .004”. Sufficient pressure to firmly position the cartridge while not so great that it leads to problems with closing the bolt or galling the lug surfaces of the bolt/receiver.

-On That note: IF your fire forming your brass with an aftermarket custom action made from 416, 17-4ph, or 15-5ph Stainless Steel it is STRONGLY ADVISED that you lubricate the lug surfaces of your bolt with a copper based anti-seize thread lubricant. Ignoring this can and WILL cause the lug surfaces of your bolt and/or receiver to work harden and gall.

PLEASE DO NOT TAKE THIS ADVICE LIGHTLY!

Once I’ve completed the false shoulder operation, I like to wash my cases. I really like the stainless steel tumbler and media kit from STM.
( http://www.stainlesstumblingmedia.com/ ) 45 minutes in warm water with Dawn Soap and a dusting of Lemmy Shine rewards me with bright, shiny, sterile cases. The washing process removes any case lube from the mouth. This will make loading the case with powder much easier because Imperial sizing wax causing powder kernels to stick to the case mouth.

Priming and powder charges:

After the cases have completely dried, begin priming. Any large rifle primer will work. You’ll need to tailor yours to what the gun wants. Federal and CCI are almost always the top choice. I prime my cases with an RCBS auto loading hand primer. It may not be sexy, but I can bang out a 100 cases in an absurdly low amount of time. Remember, this is tactical shooting, not benchrest. I do not think any advantage is realized by becoming overly obsessive about primer seating depths. Ensuring that they are firmly seated in the pocket has shown to be plenty sufficient.

For my setup, I used an old supply of IMR 4895 as my fireforming powder. Because I had a false shoulder, I did not feel it was an absolute requirement to use an elevated powder charge. This proved to be a very successful setup. 35
grains of IMR 4895 blows the case out with crisp definition when fitted with a 100+grain bullet on a case with a false shoulder.

Fireforming Projectiles:

Several shooters have reported back with exceptional accuracy during the fire forming process. It’s very possible to be competitive while building your brass inventory, do not hesitate to develop an accurate fireforming load. Velocity will be down a little, but that should in no way be a deterrent.

Bullets should have a long tangent ogive during fireforming. Even with the false shoulder method. The longer bearing surface promotes a good “sneeze” to firmly press that brass case against the walls of the chamber. Crisp transitions at the big and small diameters of the shoulder are what we’re looking for. Primers should not be flat like a manhole cover, but its ok if you see the outer radius of the primer shell tightened up a bit when compared to a fresh one.

Reloading:
The data offered here is limited to what we’ve run here at LRI. As always, this data is for reference only. Start small and work up. Live to shoot another day!
I start by first depriming and washing cases. As mentioned, I use SS media. Next, I anneal the necks using a Giraud automatic cartridge case annealer. It’s expensive because it’s worth it. I don’t have to baby sit the machine and it’s proven itself to be very predictable and reliable. Best of all, it’s fast. Time is always precious.

http://www.giraudtool.com/annealer1.htm

Our sizer die is a full length, neck bushing die. I advocate a .001” to .002” setback on the case shoulders. This is ensure the brass runs well in the rifle. Again, this is tactical shooting, not benchrest. The gun must first WORK if you are going to be competitive. After sizing, I wash the cases again to remove the Imperial Sizing wax residue. I don’t like cases with lube on them as they are dust magnets. This is hard on chambers and makes the rifle sluggish. Magazines also become lethargic because of sticky cases.

https://www.facebook.com/405264722901625/photos/pb.405264722901625.-2207520000.1431352236./799749116786515/?type=3&theater

My rifle works best with Retumbo. 49.2 grains. This fills the case to about the middle of the neck. I vibrate my cases by setting the loading block on the chassis of a rotary tumbler. The buzzing motor settles out the charges to just slightly below the neck/shoulder junction. This makes seating the bullet easier and avoids crunching powder kernels. For this powder, the best luck I’ve found is with the Federal 215 Match Rifle Primer. Standard 215’s will likely work just as well.

Bullets cover the full gamut of 100+ grain weight class 6mm projectiles. I have really good luck with the Berger 105grain Hybrid Target bullets. Some of our team shooters have moved to the 115’s. Others are using Sierra’s. The chambers were all cut with the same tooling, so it serves to demonstrate that the cartridge can be quite forgiving.

With the 105 Bergers we see velocities in the 3160 fps range when shot from 24” barrels with an 8 twist. This value is with Retumbo propellant. H1000 is what the case was originally designed for. I like Retumbo as it’s much more sterile when running a suppressor. Carbon buildup isn’t nearly as aggressive. Any SINGLE BASE MAGNUM powder that lives in the vicinity of these burn rates can be experimented with. The trick is to avoid the double base powders as these are shown to erode the barrel throats more aggressively.

I do not have data on other propellants. We only know what works best for us.
With our tooling we recommend starting with a cartridge overall length of 2.883” This works well with the 10 round AI magazines. The 5 rounders demand a shorter COAL unless modified internally. My particular rifle shot really well at this COAL so I've left it alone and stayed with the 10 round magazines. This is where you’ll have to decide what works best for your particular needs
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For seating bullets, a standard 243 Winchester seating die works perfectly. Because the case body taper is unaltered from the parent case, it aligns in the die just fine. You’ll only have to reset the height of the die on your press. I’ve made dedicated seating dies and we found zero advantage. Because of this, we only offer the FL, NB sizer die.

I hope this serves as a good primer for exploring this cartridge. It’s performance has shown great merit and the advantage of longer barrel life means more time is spent shooting a given setup rather than having to constantly “start over” when changing barrels. I’ve read where it is suggested that barrels be cleaned every 250 rounds. Part of this cleaning process includes lapping the throat of the chamber with JB bore paste. 40 strokes. No more, no less. I personally have not done this. I’m intentionally abusing my setup just to see what it will tolerate. This is an ongoing experiment so as I collect more data, I’ll pass it along.

LRI has two major chamber variances for this cartridge when using Lapua or Norma brass. We’ve tightened up the neck/shoulder radius from the original case. We’ve also experimented with the “Bore Rider” throat. This throat geometry alleges to offer increased velocity with a given powder charge. The idea is to generate a 2ndary pressure curve by stepping the land profile of the throat. The results thus far have been inconclusive. For the time being we encourage shooters to use the conventional throat geometry as it seems to give equal performance without the risk of premature throat damage.

We also offer the original chamber as designed by Joe Hendricks. This chamber is best used with Winchester brass. Winchester brass is a bit smaller than the Norma/Lapua brands. The chamber was tightened up because of this. There is ZERO advantage to running this chamber with Norma or Lapua. In fact you'll only run into problems as the case web area is too tight. We've done the hard part here so don't over think it!

Good luck and good shooting.

LRI

UPDATE:  1.26.2017:

Last season on the 6mm Comp Match was done with RL26 as the propellant of choice.  I installed a new barrel and while fire forming cases I found that 47.7 grains accelerated a 105 Hybrid to 3180fps.

UNICORN DUST!

There's been some traffic that this powder encourages a bit of fluctuation with temperature.  I personally did not experience this.  I would however encourage anyone considering it to be diligent in putting the unused portion back in the jug.  Resist the urge to leave it in a Chargemaster or thrower.  There seems to be the potential for the stuff to go "soft" if left in a humid environment for a prolonged period.