450b Chamber Design Question

[Hoot]

I do not have access to Bushmaster's CAD drawings, so I ask any experts out there (..T) to clarify for me the following:

To me, when a cartridge is associated with the expression "headspaces on the mouth", that means to me the mouth should engage some surface to limit how far it goes into the chamber. Just like when a belted cartridge's belt engages a ring machined into the chamber, or a rim engages the breach. When I was processing the last batch of .284 brass, I cut one to 1.70 inches length. I inserted it into the chamber and with the extractor and ejector out of the bolt, closed it until it locked. Upon shaking the rifle, I could hear the cartridge rattling back and forth. This piqued my in interest, so I carefully cut a series of additional cases starting at 1.710 and down to 1.701. I found I could fully close the bolt and the lugs lock up to a case length of 1.706 inches.

So my question relates to the following diagram. Is the rifle designed so that the face of the bolt fully meets the breech of the barrel at the same time the mouth of the case meets the lip of the chamber A, or does the mouth engage the lip a little before the bolt engages the breech to keep positive pressure on the case B, or does the mouth stop just short of the lip when the bolt engages the breech, relying upon the extractor to keep it from going further forward C?

Conceptual Drawing


If you've been following my reloading efforts, you probably know where this is going. I've been spending a lot of time quantifying my brass lengths and was concerned it was in vain since they shrink out of spec (1.69-1.70) quickly. If it turns out, the rifle does not rely upon case length to set headspace C, then I will not concentrate my efforts so much trying to match lengths.

Hoot

[Siringo]

Hoot -- I am not an expert (unless I am out of town), but in my carbine, I found that the extractor does not snap over the rim, until the cartridge is fully chambered and the bolt begins to rotate and lock. I had some cases that I cut to short that would not engage the extractor when closing the bolt softly. However, when was released in the normal manner -- they would snap in. There might be some bounce back in the chamber.

When you are hearing the cartridge shake back and forth -- do you have the upper separated from the lower? If not, you may be hearing the inserts in the buffer rattling.

[Hoot]

Hoot -- I am not an expert (unless I am out of town), but in my carbine, I found that the extractor does not snap over the rim, until the cartridge is fully chambered and the bolt begins to rotate and lock. I had some cases that I cut to short that would not engage the extractor when closing the bolt softly. However, when was released in the normal manner -- they would snap in. There might be some bounce back in the chamber.

When you are hearing the cartridge shake back and forth -- do you have the upper separated from the lower? If not, you may be hearing the inserts in the buffer rattling.

I like that saying!

No, I know it's the brass as I can hold the upper with the barrel pointed down, slip a rod up the barrel, into the empty case and feel it jiggle lightly. I have not measured the rod's perceived movement, but it feels like a few thousandths. Again, I'll mention that the extractor and ejector were out to allow the case to find it's natural position. I'm waiting for someone who knows for sure how it's works in theory as well as practice to wade in.

Regarding the extractor, I know what you mean. If I'm trying to hand feed an empty cartridge beneath it, sometimes it's reluctant to snap over the rim. I usually tap it a couple of times or ease the bolt in to where it begins to engage the lugs and short cycle it around that position. When I hear the distinct click, I know it picked it up.

Hoot

[BD1]

I'm not an expert either, and I don't play one on TV. However, I've had some experience with varying head space and below is the way that I've come to understand the issue.

All firearms have 'head space". It would appear that your .450B has .006 head space. Even rimmed cartridges have head space, in that case it is the difference between the distance from the face of the bolt or recoil shield to the face of the cylinder or chamber rim groove, minus the thickness of the rim. It's a small number, but it's there. .22 competitors spend hours in the evenings sorting those little cartridges by rim thickness to try and gain an edge in consistency. Without any head space your firearm would tie up after the first few shots from powder residue, or from the first tiny piece of grit. The head space allows clearance for those sorts of contaminants to get shoved out of the way.

Head space in an typical bottle neck rimless cartridge like the .308 or .223 is the difference between the measurement from the case head to the datum line on the shoulder of the case, and the measurement from the bolt face to the datum line on the chamber of the firearm with the bolt closed. Excess head space can be a real problem in high pressure bottle neck cartridges as the increasing length difference between the chamber and sizing dies causes the case to stretch out on firing. Eventually this leads to a case head separation which, when you're lucky, only leaves the gun tied up with the partial case stuck in the chamber. When you're unlucky the escape of high pressure gas into the action causes other, more serious, mischief. This is a constant issue with bottle neck cartridges as that dimension of the case is adjustable to a certain extent depending on how you set up your sizing die in the press, and the pressures involved are often high enough to cause the brass to "flow", and to compress and stretch the locking lugs over time. This can be a real issue when loading for older mausers or smellys.

IMHO, In the rimless case of the .450B the result of excess head space is more likely to be either a misfire due to the firing pin not reaching far enough to ignite the primer, or a failure to extract if the short case manages to stay in front of the extractor all the way through the cycle. The action of the sizing die is not really adjustable in regard to head space, and there's no way to "trim it longer" that I know of. if you want to see an endless argument over whether a cartridge of this design is actually held against the bolt until fired, or not, just go to any major gun forum and search ".45 acp headspace on the extractor"

Be prepared to read for a month or two.

Perhaps we should each take one case and trim it down until it malfunctions, and then compare our results. The average would give us a "too short" dimension based on as many different guns as we can get to participate.

BD

[Hoot]

Yeah, I suppose it's been beat to death among .45 enthusiasts. My concern about it is not failure to extract or ignite. First off, I believe a chromed chamber was a bad choice for this particular caliber but I understand Bushmaster's desire to keep it in the combat weapon philosophy of design. My concern is that despite the extractor's holding power, that the case still jumps forward a little. Not too much for it not to ignite, but when it does, that gap between the had and the bolt face, combined with the chromed chamber is why it seems to drive back and imprint the head with only slightly stout loads. Possibly it contributes to first fired lengths decreasing to out of spec (1.69-1.70) right away with brass that Hornady seems unable to produce at a fixed 1.70 out the door.
http://www.450bushmaster.net/viewtopic.php?f=5&t=827

I agree it would be nice to know what represents an unusable case length as opposed to one simply out of spec.

Hoot

[Siringo]

I just took a case and shortened it to 1.640 inches. Inserted a wolf magnum primer. Then inserted the case under the extractor and chambered. BANG!

Next I just inserted the case into the chamber with primer (would not engage the extractor). Click, click, click. No indentation in primer.

[BD1]

The real test would be to load a cartridge in the short case to the usual OAL and chamber it from the magazine. If it chambers and extracts that will answer the question.

AND, if it fires OK, then we can start that same 'head space on the extractor" discussion here as well, Not.

In my case, at some point my cases will get too short for the body crimp to be effective. Having gotten twenty loadings out of them, I shouldn't complain. But I really was hoping to use them like .45 acps and shoot them until I lose them, or they split at the mouth.

[Siringo]

Oh -- I didn't mention that the primer backed out .016 inches.

One can't complain about 20 reloads.

[wildcatter]

I'm not an expert either, and I don't play one on TV. However, I've had some experience with varying head space and below is the way that I've come to understand the issue.

All firearms have 'head space". It would appear that your .450B has .006 head space. Even rimmed cartridges have head space, in that case it is the difference between the distance from the face of the bolt or recoil shield to the face of the cylinder or chamber rim groove, minus the thickness of the rim. It's a small number, but it's there. .22 competitors spend hours in the evenings sorting those little cartridges by rim thickness to try and gain an edge in consistency. Without any head space your firearm would tie up after the first few shots from powder residue, or from the first tiny piece of grit. The head space allows clearance for those sorts of contaminants to get shoved out of the way.

Head space in an typical bottle neck rimless cartridge like the .308 or .223 is the difference between the measurement from the case head to the datum line on the shoulder of the case, and the measurement from the bolt face to the datum line on the chamber of the firearm with the bolt closed. Excess head space can be a real problem in high pressure bottle neck cartridges as the increasing length difference between the chamber and sizing dies causes the case to stretch out on firing. Eventually this leads to a case head separation which, when you're lucky, only leaves the gun tied up with the partial case stuck in the chamber. When you're unlucky the escape of high pressure gas into the action causes other, more serious, mischief. This is a constant issue with bottle neck cartridges as that dimension of the case is adjustable to a certain extent depending on how you set up your sizing die in the press, and the pressures involved are often high enough to cause the brass to "flow", and to compress and stretch the locking lugs over time. This can be a real issue when loading for older mausers or smellys.

IMHO, In the rimless case of the .450B the result of excess head space is more likely to be either a misfire due to the firing pin not reaching far enough to ignite the primer, or a failure to extract if the short case manages to stay in front of the extractor all the way through the cycle. The action of the sizing die is not really adjustable in regard to head space, and there's no way to "trim it longer" that I know of. if you want to see an endless argument over whether a cartridge of this design is actually held against the bolt until fired, or not, just go to any major gun forum and search ".45 acp headspace on the extractor"

Be prepared to read for a month or two.

Perhaps we should each take one case and trim it down until it malfunctions, and then compare our results. The average would give us a "too short" dimension based on as many different guns as we can get to participate.

BD

Ding, Ding, Ding...BD wins the little rubber duck again!!!!

Holy Cow! I at first wanted to stay out of this endless pursuit, but BD makes a comment that may eventually prevail. Quote.."if you want to see an endless argument over whether a cartridge of this design is actually held against the bolt until fired, or not, just go to any major gun forum and search ".45 acp headspace on the extractor"..

How to start and then not be drawn into this continuing Saga. Hoot, first of all, in some things you are just too anal, in other things, you are a methodical mad-scientist, doing the good work.

There are +- measurements standards in all of the machining world. The 45acp has a cartridge short length std of .010" and the long end of the chamber is another .010", for a total slop of .020". So, as long as the firing pin can get to the primer...

To answer you questions, somewhat (I hope I just don't open a kettle of fish). About your diagrams, it's a combination of spaces in B & C.

Wiping a dead horse is not fun to me, and we have the most accurate rifle, ever mad (according to the Outdoor Channel). What are we going to accomplish with all this Hyperbole? If my Dragster beats your Dragster by .00001 thou of a second, well, I guess I win.

My advice is to shoot the weapon and if the cases get to short after 20-50 reloads, get some new brass. A better thought, to get to that many reloads, is to anneal them often.

I'm not trying to discourage you at all, but sometimes we need to pick our fights.

BD, again claims not to be a Expert on TV, but we know better, after all he really is Steven Seagal, in real life . Myself, I was at one time a pert, having reformed, I am now an ex-pert..wink.

[Hoot]

Thanks for your informed inputs guys.
It may seem like I'm throwing rocks at the temple at times, but I just enjoy learning as much as possible from every gun I have.
When I approached this as a possible concern, I was not aware this had already been debated ad-nauseum. I am only concerned about the 450b, which is a lot more energy yielding than a .45 ACP. If you say "Don't worry about length", that's good enough for me. At 500 rounds on the trigger, I'm not looking for more investigative work along the lines of a non-issue. For the sake of velocity studies and groups, I will still sort by length, for obvious reasons. No, this is certainly not a rifle designed for accuracy, especially with such a long leade. It does have it's moments of surprising accuracy all the same. I will continue to enjoy it and possibly modify my life cycle study thread to accomplish something other than just expend the same, tired load, over and over to quantify the inevitable. They shrink, They shoot and MOA accuracy below 2 is as much a gift from the wind gods as the cartridge gods.

I'm still trying to figure out why my brass shrinks more on one side than the other? There are plenty of Windmills out there still unchallenged.

Hoot