Re: Signs of high pressure?
Pressure signs are not strictly related to propellant charge rates. As one responder says his 260 load and his friends are vastly different that I will try to explain below but first we will look at size die and chamber dimensions.
Stuck cases can be the result of two more things not mentioned by others. One is your size die may not be sizing your case down on the base dimension enough and you are getting an interference fit when it is chambered. Measure your fired case .200” up from rim with micrometer and then measure your unfired cases at same point. I run tight chambers all the time so I know what to expect but brass fired in another rifle and then in a new rifle that is not sized properly can give you as stuck case with no pressure. What happens is the case say snug up when bolt hits forward end of travel and when you push bolt knob down you get a fairly substantial mechanical advantage as it cams in and you have then force it it and since the case and the chamber is tapered you get a lock up condition from hell in some instances.
I have seen guys shut bolt and when they went to open it they had to beat the bolt up and not even have fired the round ! ! ! !
Also check your fired case to see if it is egg shaped as such will cause a tight case extraction. Egg shaped chambers are not that uncommon and the fact that the extractor holds the case off center in a big chamber only makes it worse.
Assuming it is not a die problem or you have cases fired in an oversized chamber then you most likely have a barrel internal dimension problem.
Unless you know what the land and groove dimensions are you are just spinning your wheels. This is why the manuals say start low and work up. There is no requirements as to barrel dimensions for the various barrel manufacturers we get barrels from.
There are recommendations but insofar as a requirement that barrels meet specific tolerance ranges, it is pure chance the same load will give similar performance in different barrels.
Lets say you have a 30 cal barrel and the min bore diameter is supposed to be .300X.308. I have seen 30 cal barrels run .298x.3055 to .301x309. Bullet fit in barrels is not readiy apparent unless you have a good, make that a very good bore scope.
I have examined barrels that have the bullet contacting the middle of the groove and half the diameter of the groove all the way to the muzzle. On the other end of the spectrum is I have seen barrels that show little bullet contact with bore and not even at that. Obviously if you have a load that gives you normal fired case dimensions at .300X.308 then if you have a barrel that is .299 or .298 min with a max of .307, .306, or less you are going to have a interference fit which will raise the pressures tremendously.
Barrels on the loose size you may or may not notice lower pressure as just looking at the primer is not conclusive proof of pressure.
For instance most every knows that a cratered primer means high pressure and what appears to be reverse primer flow can be achieved by high pressure as well as a tired firing pin (striker) spring that does not have enough energy to hold the primer in during the pressure curve rise and fall and thusly there is a condition where the primer wants to crawl up inside the bolt face.
This is not to say manufacturers’ A, B, and C are all different. Only the gov’t contracts have callouts for land and groove dimensions and they are checked at first article acceptance upon delivery to determine if they meet or fail the drawing requirements. When we as individuals get a barrel we don't have the means to determine exactly what we have unpacked.
I once chambered two barrels from the same manufacturer received in the same shipment and chambered with the same reamer on the same day. I headspaced both to snug closed on a GO GAGE and I ran erosion gage in the bores to record their zero round location with the same erosion gage.
The difference was 1 ½ rings more on one barrel than the other! ! ! ! ! ! ! That means the erosion gage went further into the barrel by .150” which can only happen when one barrel is larger diameter internally than the other assuming both are new.
As well I got several barrels from another manufacturer maybe eight years ago. I chambered one up and I shot it and it never really shot well. I got a very nice Olympus bore scope and finally last year ran it down the barrel and I immediatley knew why it does not shoot well.
It is a buttoned barrel and apparently it was not reamed after it was drilled. It could not be seen without the bore scope but the inspection revealed gobs of steel build up on top of the lands that just ripped hunks of bullet jacket off as it went by. Without a doubt the worst barrel I have ever seen.
But without the bore scope it looks just fine. I have another one I greased up when I got it and I am dreading running the scope down it. Or as I tell some it looks like it was rifled with a mill bastard file and a brick! ! ! !
But this is what you deal with when you buy single barrels from vendors and their QA is not real good.
Bottom line is back off your load every time you change barrels.
If you want to see how changes happen during barrel life load up 100 rounds and at new shoot 20 continuous rounds through a chronograph at same temperature. The remaining 80 rounds would be stored in a ammo can and fired at 1000 round intervals which will take you to 5000 rounds.
You will be amazed at just how much velocity you lose. With a medium case you probably won’t see much velocity change up to the 3000 round test point but at 4000 and 5000 I suspect you will see maybe 75 to 90 FPS velocity deterioration.
The rule of thumb on gov't barrels is a loss of velocity of 200 fps or more and a 150% increase in dispersion based on acceptance specs. For instance the M16 series of weapons have a requirement that they print 4.5" or less at 100 yards for ten shots. Conversely when a barrel exceeds 7.2" for a ten shot group is is considered to have failed.
Doesn't that give you a real good feeling to know the gov't will accept a battle rifle that will barely print a E silhouette at 300 yards/meters?
Kind of makes me want to puke.
![attachment.php](http://[url="http://www.mdshooters.com/attachment.php?attachmentid=36312&stc=1&d=1302632246"]http://www.mdshooters.com/attachment.php?attachmentid=36312&stc=1&d=1302632246[/url])
