I have a rifle chambered in .308Winchester. I reload for it. Should I make ammo for it using .311 or .312 diameter bullets? Provided that the case fully seals the chamber and the throat of the chamber is not radically abrupt, will this perfect safety that I then enjoy improve the accuracy of the rifle, or any other aspect of its performance? If the case does not fully seal the chamber and/or the throat of the chamber is radically abrupt, how imperfect will this make may safety?
Now seeing that you have a post count over 7000 makes this painfully obvious you are being sarcastic. However, I will bite.
"Should I make ammo for it using .311 or .312 diameter bullets?"
Should you? Probably not. It would not make any economical sense to do so, as you would likely need custom dies to sufficiently expand the neck and would severely limit your choice of bullets. Can you? Absolutely! The bullet will swage itself down down to whatever your bore/groove size is and will come out the other end without any problems. that extra .004 will effortlessly squish down through the throat without a hiccup.
"Provided that the case fully seals the chamber and the throat of the chamber is not radically abrupt..."
Let me begin by clarifying some elements of my previous statement. From the tone of the question you seem to take issue with the chamber seal and the abrupt throat. The chamber seal relates to how the case fits in the chamber, ultimately if the case seals the chamber, all the pressure is going down the barrel, and anything in front of the pressure is going with it. This is why it is safe (though inadvisable for the health of your barrel) to fire .243 Win, .260 Rem, and 7mm-08 through your .308 chambered rifle. Because the case properly fits the chamber, it will seal and the bullet will go downrange. As to the abrupt throat, I'm basically saying that instead of having a gentle 5 degree throat, you instead go for a 90 degree one. A five degree throat will gently and safely swage down any bullet weight that you could load into a .308 (or whatever you given cartridge is). However, if you decide to throat you rifle with a 90 degree shoulder (say trying to fit a .338 federal or .358 winchester), unpredictable things may occur.
"...will this perfect safety that I then enjoy improve the accuracy of the rifle, or any other aspect of its performance?"
It is perfectly safe, as I have shown above. Will it increase accuracy? Well if you intend on using widely available .308 bullets instead of uncommon .311/.312 bullets in you .303 British rifle, it absolutely will increase accuracy. Less gas escape and less chance for the bullets to cant in the rifling is always a good thing. Also, if you do decide to use .311/.312 bullets, you will guarantee a perfect gas seal, especially if you are using surplus ammo of varying quality. As far as performance gains go, i doubt there is anything gained (other than wider bullet selection), but you will certainly not see any performance losses.
If the case does not fully seal the chamber and/or the throat of the chamber is radically abrupt, how imperfect will this make may safety?
It can be drastic, as I am sure you know. Mixing ammo can be dangerous, depending on the variables. You never want to fire a smaller case with a bigger bullet in a large chamber with a smaller bore. An example would be using a .308 in a .270 Winchester. In such said example the bullet will jump out of the case and lodge in the throat, allowing powder gases to build pressure and potentially destroy the gun and severely injure the operator. in this case, the .308 case did not fully seal the chamber, and allowed gassed to escape and build pressure. This scenarios is not possible when the bullet is seated in the throat and the case fills the chamber completely (with obviously a few thou of expansion). However, with a long enough throat one could safely load and fire a 30-06 in a .270, provided the 30-06 round is loaded with an identical charge and bullet weight as the .270. Just as in Ackley's test, that same principle applies throughout. As long as bullet weight and max charge combination is not exceeded, the rifle is safe to fire. As to abrupt throat angle, the .308/.270 example perfectly illustrates this. If the bullet smashes into an abrupt cone (the shoulder of the .270 chamber), it could very well lodge istelf there and spontaneously disassemble the gun. Gas is lazy and takes the easiest route out, whether than means swaging the bullet down the bore, or wedging the bullet in place and rupturing the casehead.
So that's my case. Tear in as you see fit. The theory is sound and has been proven by the most reputable P.O. Ackley. If he can safely swage a .35 down to a .30, anyone else sure as hell can swage a .312 to .308.
I'm not sure if a .303 bullet will fit a regular 30 caliber barrel, but if it does it definately makes the hunt for a barrel easier.
