This is a discussion on Boat-tail bullets versus standard bullets within the Ammunition forums, part of the M14 M1A Forum category; My Devine rifle built in 1972 prefers standard Federal softpoint hunting bullets(not boat-tail). Is the barrel/chamber for M14/M1A of the early 70's developed for this ...
My Devine rifle built in 1972 prefers standard Federal softpoint hunting bullets(not boat-tail). Is the barrel/chamber for M14/M1A of the early 70's developed for this square based bullet .....or is my gunsmith friend got a better answer: Boat-tail ammo doesn't perform as well as the earlier bullets because of throat/chamber erosion in these shot-out barrels. I think the rifle has had about 4700 round thru it. The square based 150 gr bullet gets me 1.25" @ 100 yds and a Hornady match 150 gr boat-tail is 2.50".
If you don't measure the wear to the throat and muzzle you won't know how much wear you have.
I'd say the difference in group size is more likely a "preference" your rifle has for the bullet/powder combination rather than a wear issue.
I used to have a hunting buddy who bought a brand new Winchester M70 in 30.06. He was really disappointed in every factory load he shot (Fed, Rem, Win) in bullet weights from 110 through 220. None of the reload combos we tried worked either (hand sized 3 shot groups at 100 yds.) I had some left over stuff from my father in laws Rem. 742 reloads. A Sierra 180 grain flat based spitzer bullet with Fed. BR primers and IMR 4350 powder that we found in the back of the gun cabinet drawer. With that single load his rifle would shoot a dime to nickle sized 3 shot group at 100 yds. Two bullets would tear a slightly larget than one hole group and the third bullet would leave a sliver of paper between the hole it made and the hole the other two rounds made.
Sometimes, some rifles have "issues". These issues may be possible to cure by other means if you can find the right gunsmith but sometimes the right load (whether factory or reload) can make a world of difference.
Many worn barrels perform better with flat-base bullets - a fact widely known by Lee Enfield shooters. There is no ballistic advantage to BT bullets at ranges shorter than 300 yards anyway. They are slightly easier to seat during loading, though.
FWIW, the 7.62X51mm NATO cartridge was loaded with BT bullets from it's inception, AFAIK.
From my limited experiences, just seem to be a preference of the barrel. Some barrels just prefer a flat based bullet. I agree with the previous poster, until you get beyond 300 yards, any flight advantage of the BT of flat based bullets is pretty much mute anyway.
Boattail bullets don't settle down (go to sleep) at short ranges. That's why all of the 100/200 yd benchrest shooters use flat-base bullets. You would have to go past 300 yards to see the difference -- that's where boattails start to work.
I too have read that boat tails will cause more erosion than flat based bullets. In fact the information was in a book written by a ballistician of some repute. The book was written back in the 1950s entitled "Principles and Practice of Loading Ammunition" by Earl Naramore. He made a simple comment, but did not back it up with any proof or data, to the effect that greater erosion was probably due to the gases being forced in to the wedge shape between the chamber and the tapered base of the bullet.
Personally I have never seen actual tests done on the wear so until that happens I tend to not believe that it would be a problem in any standard rifle. Magnums with large cases and fast bullets and small calibers pushed to hyper-velocities might show this problem and I wouldn't be surprised if it were proven to be true but with our rifle pushing standard bullets (up to 168 grains) at standard velocities probably wouldn't wear the barrel at any appreciable rate using boat tails vs. flat based.
That same book and some others (Bryan Litz's book "Applied Ballistics For Long-Range Shooting second edition) give theories as to why flat based bullets work as well, and in some cases better than, boat tailed bullets over the short range. Specifically it has to do with the jet of gas that escapes from the muzzle at the time that the bullet is ejected. That gas jet pushes on the base of the bullet enough to actually accelerate it for about 15 bullet calibers away from the muzzle. That's about 4.5" from the muzzle with the .308" caliber.
So the gasses have enough force to push the bullet faster for about 4.5" after it leaves the muzzle, that means that anything that changes where that gas pressure is concentrated can influence the direction of the bullet's path, to some degree. I say to some degree because the spin of the bullet will tend to make the bullet return to the path of it's center of gravity (or is it center of spin? well, one or the other). Any time a pressure is applied against a flat surface it is easier to keep the force against the surface over a larger area than a smaller one since slight angular differences wont move the force off of the flat surface.
Taking that in to account, it stands to reason that a flat based bullet will experience a more consistent force than a boat tailed bullet. Not only would the force be more consistent in magnitude but in direction too (assuming a proper shape to the crown). Bench rest shooters use this knowledge to their advantage by pushing their muzzle out over the edge of the bench that they are shooting from. They do this so that the gas jet wont be reflected back toward the base of the bullet after it bounces off of the surface of the bench. I had a bench rest friend prove to me that this works with his 100 yard bench rifle. It didn't make a huge change (a couple of tenths of an inch) but he was using a tiny 6mm bullet and he was able to recreate the change in group size every time he moved the muzzle. I haven't tried to prove that it works for our rifles but I do keep my muzzle past the edge of any bench that I shoot from. An accumulation of lots of little tricks will eventually add up to some pretty impressive results.
So over the short distance (out to 300 yards) I think that the flat based bullet is a good choice, but when the bullet moves in to the medium (300 to 600 yards) and long (beyond 600 yards) ranges, the boat tail's streamlined shape is more advantageous because air friction has more of an impact on the ballistics of the bullet. At short range the bullet has enough energy and momentum that it just muscles it's way through the air, at longer ranges it hasn't got the muscle any more and it has to take advantage of anything it can to perform well. At short ranges the base matters because it is how the bullet receives energy/muscle. At longer ranges the base isn't a source of energy, it's actually a source of friction, it drains energy away from the bullet due to the way the air moves over it. With a flat base you get tiny curls of air that cause a vacuum at the base and the bullet slows down quicker and begins to buffet. With a tapered boat tail shape the air moves more smoothly and flows without those tiny curls.
Sorry for the long winded post (I'm infamous for that) but I love to talk ballistics and hopefully it helps somebody pick up information that improves their accuracy.