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Discussion Starter · #1 ·
Can someone help me? I have been told that this is the best firearms forum on the internet. I live in the UK and have an extremely healthy interest in ballistics but because of my location I do not have a great deal of access to actual firearms. As a result, I feel that I may be missing some vital experience that is making it impossible for me to resolve a problem that has been bothering me for some time.

Recently I have been intrigued by the 8.6 Creedmoor calibre based upon a modified 6.5 Creedmoor cartridge case with a .338 calibre projectile.

Now having researched some other 'similar' large bore subsonic calibres I found the .338 Whisper cartridge that came in two versions, one based upon the .221 fireball and the other based upon the 7mm BR.

So here is my question. According to all the information I have been able to find, both the 8.6 Creedmoor AND the 7mm BR (Whisper #1) cartridges require an AR-10 platform with a .308 bolt face and a .338 calibre barrel.

So assuming that I had an AR-10 with the .308 bolt face and a .338 barrel conversion, would that rifle fire BOTH 8.6 Creedmoor AND Whisper .338?
 

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No...
 
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Discussion Starter · #3 · (Edited)
Thank you, and I appreciate you taking the time to answer but can you please explain why. I made this post because I openly admit I am not as knowledgeable about these matters as other members here and I am anxious to learn what I am missing.
 

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Ancient Gaseous Emanation
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Chamber dimensions preclude interchangability.
 

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Discussion Starter · #5 ·
Okay this was what I suspected but as I explained, having never actually owned or built a rifle of any sort I am still at a loss to find exactly which dimensions you are referring to here.

I appreciate that with a calibre change from 5.56 to something else there is often a need to change the bolt, magazines, barrel etc but with numerous conversions for instance to .300 blackout, it is stipulated that only a barrel change is required as the bolt face, magazines etc all work with the new calibre. Infact many people go to great lengths to mark specific rifles with tape etc to avoid incorrectly loading .300 blackout into a 5.56 rifle because as I understand it the rounds DO chamber but obviously the larger calibre bullet will not be able to leave the barrel causing a serious safety issue if the weapon is fired.

With the example I have quoted namely the 8.6 Creedmoor and the .338 Whisper, BOTH calibres can supposedly be fired from a standard AR-10 using the .308 bolt face and the dimensions of both cartridges are such that they work with standard AR-10 magazines. And in this particular case since both cartridges use a .338 projectile we would be using a .338 calibre barrel.

I assume (but I don't know) that the dimensions within the chamber that might cause issues have something to do with the shape at the shoulder of the cartridge and the entrance to the barrel which presumably is machined to match this. However I was given to understand that the .308 was the base parent cartridge for BOTH rounds albeit that both are derived from cartridge casings that are already modified (i.e. necked down) from the original .308.

I understand that this may be obvious to you and perhaps frustrating to explain to a newbie. I do not expect to be spoon fed but I have tried and tried to get to the bottom of this but with the limited information I have on both calibres I am struggling to find out where the differences lie. I know the .338 Whisper #1 is based on the 7mm-BR casing but I am still being told that that itself is still derived from the original .308 cartridge case, hence the reason it works with the .308 bolt face.

I really would appreciate being educated on this!
 

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The chamber is the back end of the barrel that holds the round (brass and projectile) before firing and the brass after firing. The chamber dimensions must exactly match the round or interesting things can happen.
 

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223.jpeg

300bo.jpg

You actually picked a good example and to expand a little more. The chamber is the beginning portion of the barrel and the case like popeye said must fit correctly. If you look at the 300 blackout picture you'll notice the case even though it fits in the chamber it is not tight in there like the .223 case it. Lets say the 300 black out had the same diameter bullet as the .223 and would physically fit down the barrel. If you were to fire that case the extreme pressure present in the case would expand the brass out to the chamber, possibly rupturing and worse possible allowing that pressure to escape and essentially blowing up the gun. Sometimes people who have fired the wrong cartridge in a gun get lucky and the case just expands, splits and gets a little gas in there face, other times the gun blows up and can injure the shooter. Also depending on the case sizes between the two different cartridges many times the case won't physically fit allowing the gun to go into battery, other times they are much too small not allowing the firing pin to strike the primer and other times they fit just enough to cause disaster. There are some where you could probably get away with it but you wont have any accuracy like firing a .270 in a 30-06.There are some guns designed to shoot multiple cartridges but all the ones I'm aware of will be straight wall cases and i think they are all revolvers. Examples are some 45 colt,schofield or auto rim revolvers can shoot 45acp with moon clips only if it's designed to. Not all of them will. Other i can think of are the S&W 460 that can shoot 454 casul and 45 colt or some 45 colts that can shoot .410 shotgun. Again not all guns chambered to do this. Not all 25 colts will shoot .410, just ones designed to.
 

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Here is an example of firing too small of case in a chamber. This actually happens quite often with firing 9mm in a .40 shooting glocks because some of the mags are interchangeable between two different guns.

 

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Discussion Starter · #9 ·
Thank you for the far more in depth answers and I appreciate that my original question i.e. can both cartridges be fired from the same weapon has been answered but I guess what I am searching for now is the reason WHY?

I fully understand the examples given and I am well aware of the issues with 5.56 and .300 blackout as well as 9mm and .40 S&W but in all of these cases we are talking about issues relating to two DIFFERENT CALIBRES being chambered in the same weapon with the same barrel.

In the specific example I have mentioned we are talking about two different wildcat cartridges BOTH of which have a .338 projectile and a case based upon the dimensions of the .308 Winchester and BOTH being fired from an AR-10 with a .338 calibre barrel.

Now, I have no doubt whatsoever if the far more knowledgeable collective has stated that these two cartridges cannot be fired from the same weapon then that is the case. I do not think you guys are wrong. I'm just trying to ascertain where that difference in the case dimensions exist because I cannot find it anywhere.

Examination of the .308 Winchester cartridge shows it to have a 20 degree shoulder angle whereas the shortened and necked up 6.5 Creedmoor case used for the 8.6 Creedmoor has a 30 degree shoulder angle.

So based upon the photos given above a barrel designed to chamber this cartridge would need a matching 30 degree angle and headspacing etc designed to ensure that the round seats properly in the barrel/chamber.

What I cannot find information on however is once the 7mm BR case has been necked up to accept the .338 calibre bullet what are the 'rogue' dimensions that tell us it will not chamber in the same rifle. Shoulder angle? Full round length? Is this a case of one cartridge being longer and the other shorter creating a headspace issue or is there something else that I am missing?
 

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The problem is case dimensions. Shoulder angle wont matter if one case is longer than the other. meaning one would not chamber and the other would be much to short to headspace off the shoulder. Just looked up some case dimensions and can't find a whole lot and the pictures are in the wrong format so I'll type it out. 8.6 has a rim diameter of .473, the 338 whisper is .469. case length on the 8.6 is 1.685 and the 338 is 1.499, that is a huge difference. You might be able to put a 338whisper in a 8.6 chamber but the case will probably blow out and the 8.6 is much to long to fit in the 338 chamber. When a round is chambered there are very tight tolerances measured to the thousandths of an inch from the inside of the chamber to the outside of the case. When you fire a round there is 10's of thousands of pounds of pressure in that small chamber and that pressure is designed to propel the bullet down the barrel. if that pressure has somewhere else to go it can be devastating. From the very least if will destroy the case, to the very most it will explode the gun and can cause personal injury to anyone close by. here is a pic of a 30-06 case next to a .243. I wanted to use a .308 because it shares the same bullet as the 30-06. A .243 case is relatively close in size to a .308 with the neck diameter being a lot different. In other words picture the case on the right as a .308. That will fit in a 30-06 but when you fire it(if it were to go off) the case would try to expand to the same size as the case on the left. much like those .40 caliber cases did in the video above. Like i said you're talking 20 to maybe 50,000psi. Also obviously the longer 30-06 will not even fit in a .308 chamber because it's too long. It only takes a few thousandths of a inch difference in a chamber to start causing problems.
20190915_092554[1].jpg
 
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You may be overthinking it a bit. You've hit the nail on the head yourself when you said,

So based upon the photos given above a barrel designed to chamber this cartridge would need a matching 30 degree angle and headspacing etc designed to ensure that the round seats properly in the barrel/chamber.
If you look at the dimensional schematics for cartridges, you'll see how complicated they are, like the drawings to build a jet airplane! Here's a search. There are some minimal tolerances to be had but they are quite minimal. Dimensions of all cartridges are quite well engineered.

To answer your question about "why?" consider that the brass cartridge and the chamber of the rifle actually work together to contain and to direct the force of the burning gunpowder behind the bullet. That they have to work together means that the design of each is meticulous and deliberate to maximize performance, within the limitations of physics, of course.

A mismatch is catastrophic. if you have a gap between the brass and the chamber wall, then the brass is doing ALL the work at that point which is what causes a failure.

--Wag--
 

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Also if you're talking a AR platform and you want to fire both of those cartridges just get a upper for each one and it will work. You may or may not need different mags too but the lower should be the same, problem solved and it takes less than a minute to swap them.
 

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Discussion Starter · #13 ·
Thank you. That was the information I was missing. That's very interesting though. When I started examining the ballistics of both cartridges I saw that the 8.6 Creedmoor although having some great advantages actually appears ballistically inferior to the Whisper #1 especially subsonic. I thought that might be due to the 8.6 Creedmoor having a smaller case and therefore less powder but from what you have written it appears that the 8.6 Creedmoor actually has more case capacity. Something else I need to start researching now, although given that 8.6 Creedmoor is very much in development, I doubt that many loads have been finalized yet and there may be room for improvement.
 

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I don't really get much into many new cartridges that come out or wildcats and actually never heard of either. The 338 whisper has a wiki page that shows all the case dimensions but I'm guessing the 8.6 is really new being there is little out there. I found one pic that had just a couple of dimensions but not like most cartridge diagrams have. I'm sure it's out there somewhere i just didn't feel like checking a ton of sites.
 

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Discussion Starter · #15 ·
No there is very little on the 8.6 Creedmoor and most of the stuff out there is a year old. It was supposed to go to market this year with backing from Hornady for a commercial load. The lack of any new information suggests it's been put back or maybe even abandoned. The lack of ammunition choice for wildcat rounds like this is what hinders them and let's be honest, subsonic 'sniper' rounds have a somewhat limited market.

8.6 Creedmoor was supposedly 'invented' a long time ago and seen as a big brother to .300 blackout offering both supersonic and subsonic loads that would cycle in the same gun giving all the advantages of .300 blackout with better terminal ballistics.

And "supposedly" the 8.6 Creedmoor supersonics outperform .308 out to 300 yards with the same or less recoil.

Interesting from a theory crafting perspective which is all I really get to do, but from a practical standpoint, I am sure that most members on here will say there is nothing it can do that a lot of existing calibres don't already do unless you specifically need supersonic and subsonic capability with the same gun and .300 blackout doesn't pack enough punch.

Personally, if I was going to do a subsonic hog gun, I'd probably go the whole hog (pardon the pun) and just do .510 Whisper BUT I suppose the option of a semi-auto AR-10 based rifle as opposed to a bolt action might have some merit in certain applications.

Overall, however these seem to be niche military application rounds designed to step up terminal ballistics where .300 blackout doesn't hit hard enough and .308 subsonics don't cycle semi-autos without an adjustable gas block.
 

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My initial reply was because I was not familiar with either cartridge and a little birdie told me you were flying from forum to forum with the same question. Your best answer doesn't come from a forum it comes from real research. I understand the fun in finding the best cartridge and other musings on paper as I do it myself. I also learned real early that what's on paper only proves something when you can make holes exactly where you want them on paper or see what a bullet does when it hits a concrete block or bounces off a piece of glass. I also discovered what's claimed to be achievable on paper I may not be able to achieve.

The problem with new cartridges is the lack of information from reliable sources. SAAMI doesn't list those cartridges but you can find chamber dimensions from various other sources. I get the impression that you don't fully understand what a chamber is. I recommend you do more research on the basics. You said you weren't looking to be spoonfed but it appears you may have to be.

A quick search would tell you as already said that the OAL is significantly different. Freebore and leade comes to mind as topics you should also look up if you're not familiar with them. With slightly varying parameters as already discussed and also the real dangers already discussed you need to be careful and proceed with caution. If things go bad I'm not sure if you would post your findings with your one good hand or finger. That's assuming you can see with your one good eye or you learn to use a Braille keyboard.
 
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