Gents,

Let me preface this by saying that I am not (yet) a reloader, so I don't have all the tools and measuring devices that would be helpful for assessing this. Nonetheless.........

I have a new-to-me .243 Winchester in a Model 70 SS FWT short action CRF. This is one of teh earlier versions.

I have just started shooting it with 3 different brands of ammo, and it does not shoot great. Thus far, I have managed a couple of MOA groups, but oddly, these seem to come when the bbl gets pretty hot. mostly, it is going about 1.5 to 2 MOA. Not as goos as I would like to see for a .243.

So, I noticed that the cases were fairly "bulged"withing the chamber. More so than any of my other rifles. I have attempted to post some pics below. In them you can see the bulge in the fired case which mics about .006 in (in diameter) over the case diameter just outside the chamber at the case head. All three brands of brass show this. It is quite visible and palpable. You can see inthe one pic on the Winchester brass where the three fired cases stick out of the holder as they are so increase in diameter that they don't slide down.

I've compared cases from my .270 and 30-06, and they seem to range from about nothing (270) to about .003 over (30-06).

So for you experts, whatcha think?

Is this chamber likely oversized? If so, this will kill my accuracy potential, yes?

Thoughts and opinions appreciated!!!

Bob

http://i690.photobucket.com/al...ckbart2/DSC04241.jpg

http://i690.photobucket.com/al...ckbart2/DSC04243.jpg

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No. What you are seeing is typical and is the result of dimensional differences between the chamber and the brass. There are dimension standards the gun industry must adhere to when producing guns, and the ammunition manufacturers follow a similar set of standards so that the ammunition they produce will fit. With regards to the bulging of the brass, you are seeing the result of what happens when you insert a minimum standard case into a maximum size chamber. Nothing unsafe about it so long as you stay within the pressure guidelines of the cartridge.

Part B:

Because of gravity, when you place an undersized round in a slightly oversized chamber the round has a tendency to lie on the bottom of the chamber placing the cartridge and bullet at an angle to the bore. When you fire the gun, the bullet can enter the bore at an angle. The bullet, being slightly off center, will form a new axis on which to spin, and when this bullet exits the barrel, it will 2 axis' fighting for control which does nothing for accuracy.

One way to lessen this would be to fireform the brass and neck size, paying particular attention to maintaining very minimum headspace to help hold the case in place. This may reduce the slop to a manageable amount

I had a similar experience with my AR15
It had a 1:7 twist barrel I was shooting 55gr UMC rounds and it bulged the cases bad.
Turns out that the fast twist barrel was creating high pressure due the the loads being designed fo a 1:12 twist barrel.
I switched to 62gr bullets loaded hot for a 20" barrel @ 2900 FPS avg and no sign of over pressure
Shoot the same 55gr and bulged cases and flat primers every time
Just something to think about

yup,
because there are lots of automatics around in .243, it happens that case dimensions are kept to a minimum diameter to be sure all ammo will load reliably in anny gun and if you happen to get a rifle with a chamber cut on the large side of specs( not uncommon in mass production), that will be the result.
as said above, no problem, safety wise and neck sizing is the remedy. just be sure to keep those cases with that gun, if you have more than one .243.

Westpac made a pretty good explanation of your situation. While certainly not unsafe, it is nonetheless somewhat disconcerting and cannot be conducive to good accuracy.

Here's how it happens: The manufacturer wants to get the maximum number of chambers out of a given chambering reamer. Since the reamer wears a little smaller with each use, the manufacturer wants to start with the reamer as large as possible (I suspect that they may actually fudge above the maximum allowable dimensions of SAAMI specs), therefore allowing them to ream as many chambers as possible before the reamer has to be discarded. It sounds like you got a barrel chambered with a fresh (oversized) reamer.

Conversely, ammunition manufacturers tend to dimension their brass as small as possible, and again, I suspect sometimes they go a tad smaller than SAAMI spec, so that no gun has difficulty chambering it. You can bet that if a typical consumer picked up a box of Hyper Velocity Remchester Whiz Bang ammunition and it offered just a tad of resistance entering the chamber, well, Mr. Consumer would never buy that brand again and would bad-mouth it to all of his buddies. Therefore, factory ammunition tends to be a tad on the small side.

When you combine large chambers and small ammunition, you get what you've got. As has been said, while the poor fit of the cartridge in the chamber may not explain what you feel is unsatisfactory accuracy, the only way to find if chamber fit is the culprit is to use cases that have been fireformed to your chamber, then neck or partial-sized only. BTW, M70s of the era of your gun are not regarded as being keenly accurate. You may have difficulty getting it to shoot a great deal better that what you are currently experiencing.

I also have one of the SS featherweights. The previous posts explain the ammo situation very well. I assume your gun has the factory synthetic stock not the Macmillan that was factory on the push feed model for a while. I could never get my gun to shoot well. Sometimnes it would shoot an inch , mostly it wouldnt. I eventually relized that the synthetic stock was the problem. It was actually quite flimsy, and although free floated would allow the stock to contact the barrel when I held it over the bags. Naturally this destroyed accuracy. It was particularily soft when the outside temperaturewas hot. The solution was to buy a used wood stock. The gun turned from a two and a half inch gun to a one inch gun. The occasional one inch groups you are getting means the gun is capable but unless the gun is held exacly the same for each shot there will be different pressure on the barrel which affects the accuracy. By the way I love the classic winchester with the control feed.

FYI
Unless hand turned, a dull reamer will normally cut oversize on diameter not undersize as you would think. It only comes out to be .001" to .0015" larger in a worse case scenario. This will not effect the headspace to any real degree since this is controlled by the depth of the reamer. The shoulder will still come out extremely close to the shape of the shoulder on the reamer dull or not.

Thanks for all the replys, fellas!

You all have basically confirmed my thoughts and suspicions as to what was going on here.

The neck sizing experiment makes a lot of sense, however, since I am not set up for this stuff it is a moot point.

Assuming the chamber is a little on the large side, but within spec, would setting it back a thread help here or probably not?

TIA