Home Forums Hunting & Shooting Ask The Gunwriters A NEW CARTRIDGE DESIGN

I have been doing some research into the merits and foils of the hypervelocity .224�s and 6MM�s (i.e. .224 TTH, .240 Hawk). I am particularly intrigued by the prospect of lightweight High BC bullets in the 4,000-4,200 fps range for varmints, targets and deer.

My research indicates that the .220 Howell is likely the best available option in the .224 bore size and I will likely construct one of these critters (Between rebuilding my Rifle inventory, student loans, and catching up on my educationally postponed hunting trips it looks like I�m going to need a mint the size of Ft. Knox to pay for it all ) (God, but its hard being a gun and hunting nut in the 21st century ). However,nothing I have found so far in the 6MM flavor fits exactly what I am looking for (100-105 grain bullets at 4,000-4100 fps). This said, I suppose I will have to design and build my own.

Now, how does one begin this ordeal . . . uh. . . I mean project? I guess a likely place to start is with a parent cartridge. The 6-06, 6-264 Mag, and 6-284 have all been tried and a 6-WSM or SAUM necked down is surely in the mix somewhere. If I am going to burn gross weights of powder, expend thousands of bullets, and melt down enough barrels to fund my gunsmith�s retirement on the French Riviera I at least want to be original.

Thus, I think I will begin with the gargantuan, powder devouring, throat eroding 7MM Remington Ultra Magnum Case. Let�s shorten this monster to 2.6995 (for originality and it�s about half way between the 7mm Wby and 7MM STW) and make the shoulder angle 44 degrees (a tribute to one of the great gun men Phil Sharpe who used this angle on the 7x61 S&H).

Next, let�s fit a 31 inch (a rather unusual length to suit my peculiar personality ) #5 Contour (insert favorite barrel maker here) to a single shot 700 Remington (because of all the aftermarket bells and whistles available) action with a match grade trigger and synthetic (insert favorite maker here) stock. Top this booger with 6.5-20 30MM tube Leupold Vari-X III.

Okay, how should I proceed on this rifle making odyssey? I need your input and experience on having reamers made, loading dies made as well as your suggestions on any talented gunsmiths who are crazy enough to build this thing and not have me committed for ballistic heresy. Also include your thoughts on actions, barrel makers, stocks, triggers and muzzle brakes

Be sure to share your thoughts on reaching my velocity goals, case design and which powders, bullets, etc I need to use. And definitely don't forget your ideas for a name for this creation.

Thanks for the input,
HBB

Well, good luck on your probably impossible goal of 4000 fps with a 100-grain 6mm bullet. The biggest 6mm cartridge of which I am aware is the factory .244 H&H Magnum. Originally developed in 1955 in the middle of the 6mms' heyday, the .244 H&H claimed 3500 fps with a 100-grain bullet. But Speer reported that the Kynoch ammo only delivered 3276 fps in their test rifle with 26" barrel. P.O. Ackley wrote that the .244 H&H showed 2 inches of throat erosion after just 115 shots.

In 1991, Bill Davis tested the .244 and wrote of it in the March issue of American Rifleman. He pressure-tested the data in both 24" and 32" barrels, and was able to attain over 3800 fps in the long barrel - BUT! He discovered that throat erosion was so severe that after 150-300 rounds he could not safely attain velocities above 3683 fps even in the 32" tube. Pressures were held to 64,300 psi max. The cartridge ate the barrel so fast that the throat became too rough and abraded the bullet jackets causing restrictions - this is what caused the pressures to soar.

From this and other testing it seems to be that the 6mm caliber is limited to ca. 3400 fps with 100 grain bullets in any practical sense. An alternative is the .257 STW. This will not get 4000 fps with 100-grain bullets, but can get 3600-3800 fps at sane pressures and reasonable barrel life. This makes sense, since the .257 bullets have eleven percent more bullet base area for the gas to press against - this is what actually accelerates the bullet. If a max-effort .257 can't get 4000+ fps from a 100-grain bullet, then there is no way that it can be done with a .243" cartridge.

My advice: forget 4000+ fps and go with a .240 Weatherby or 6mm-284 - these can hit 3300 to 3400 fps without the bore erosion problems. But it is fun to push the envelope, so once again, good luck! <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />

ASSuming you can reach those velocities, where are you going to find a bullet that will hold up on deer? You may have trouble finding a bullet that will hold up in air!

Bullets should not be a problem - remember he intends to shoot at long range, where the bullet velocity has dropped off. Still, Barnes X-bullets will stand up to these velocities and give good performance on deer-sized game at close range. Nosler Partitions may hold up fine too, but I don't know for certain.

Nosler Ballistic Tips hold up fine at extreme velocities in air - I've shot hundreds of 55-grain BTips in my .243 at over 4000 fps, where they give outstanding accuracy out to 385 yards ( the longest distance at which I've actually shot them on paper ).

Thanks for all the input. It has led me in some interesting new directions in my ballistic contemplations. Since the prevailing wisdom on the magnum size case indicates its probably too much of a good thing how about this:


Use the .30/06 case with a 44 or 45 degree shoulder( very similar to the ICL wildcats of the 1960s), 30-32 inch barrel and reduce the bullet weight to 80-90 grains ( I.E. Barnes, Nosler Partition or other premium "high impact" 6MM bullet). This case should be much more efficient with the available powders and barrel wear. while still substantial, should be less of a problem.

Also do you all think it would be possible to sharpen the shoulder on the .30/06 to as much as 60 degrees? What do you think is the likely gain over the Ackley/ICL shoulder? Maybe 150fps maximum?

Again thanks for the input.

HBB

SSK industries has a line of cartridges based on the '06 with a 60� shoulder, but they are used for single shot actions.

Don't expect much additional velocity by simply changing the shoulder angle. What matters is the case volume, and the typical Improved case only gains ca. 100 fps over the parent case. Unless you increase pressures substantially, you shouldn't expect to get much more velocity, since it is really the amount of power you burn which determines the ultimate velocity. Minor changes in case volume or shape don't mean much. What really caused the increased velocities quoted for the IMP cases was the substantially increased pressures that the cartridge developers used. Safety was a secondary concern after looking good.... <img src="/ubbthreads/images/graemlins/ooo.gif" alt="" />

Sounds to me like a .257 Banshee would be to your liking. A 6.5x68S RWS case with an improved 35 deg. shoulder and blown way out. Creator claims 4000 fps from his 30" Lilja with 100 gr. Ballistic Tips and 85 gr. Fowler LoDrag's. Just over 3900 with Fowler's 110's. Another option for you to look into. >> klallen

HBB,
If you want a hot 6mm, check out these 2 links to other threads here. The 240 Gibbs is pretty hot and may fulfill your desires.
Theory and paper punching

Field Results

Thanks for all the input. It is really helping me narrow the field down.

On the .240 Gibbs, how hard is it to form the brass?

When I was a kid ( early-mid 1970s) my uncle had a .240 Gibbs on a pre-64 Winchester action with a long heavy barrel that was death on Virginia groundhogs to over 500 yards. As I remember he had one of the case forming tools for it. Does anybody still make this thing? If not what are the odds of finding a good used one or having one built?

Thanks again,
HBB

Forming the 240 Gibbs isn't terribly hard. I use new 280 brass and neck size it to 264 first, then in the 240 die I push the secondary shoulder back until it is a tight crush fit in the chamber. I have the sizer set for the actual headspace plus .002". For the fire form secondary shoulder I place a 0.008" spacer under the lock ring. Large pistol primers, 14 grains of AA #2, a case full of cream of wheat and voila you fire form. I full length size then, trim and sort through the rest of the case prep details (primer pocket uniform, flash hole debur, weight and concentricity culling). All told I worked 200 pieces of brass over one afternoon and lost only 6. Dies are available from CH4D, Hollywood, and others. I bought my set on gunbroker for $35.