Bump up.

I know this pic is a bit late, I recollect a posting before in relation to the ability of boat tails to seal the bore versus flat base projectiles.

I did an article on one of the forums years ago, but know with being stuck in the big wet, with nothing to do and limited info I came across this pic showing base bump on a 2 groove barrel, the seal created by the bump above the boat tail and the extrusion of the lands.

If the boat tail projectile is pushed above 2000fps, you will get adequate sealing.
Attachment 113986

Thanks Muffett it clearly shows it, in relation to the bumping I thought with the boat tails you suffered more blow by at the initial stages when the projie leaves the case prior to it starting to being engraved by the rifling I am pretty happy with the way my rifles go using boat tails. But have brought 200 Taipans as I shot them years ago when Malcolm Bone owned it and they were a superlative pill being a .312 FBHP so looking forward to range time again sadly my xcel spread sheet decided to go into orbit with all my 303 DOPE for all my 303's years of loading and range time lost in cyber space. I will now employ the time honored principal of pen/pencil & paper.

So why do they key hole in some rifles when the OB don't.
I know I will crawl back under my rock.

Quote:

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Originally Posted by Bindi2

So why do they key hole in some rifles when the OB don't.

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What do you mean by OB, Bindi?

O B = open base. The Mk7 is a straight OB .. The Mk8 is a OBBT.. To be perfectly correct FMJOB and FMJOBBT.

Bindi,

Thanks, clear now. TLA*s get a little confusing unless you're "in the know" so it helps to ask.

I'd suggest that the reason that Taipans, which are FMJ and therefore base closed, will continue to keyhole in some barrels, is because the obturation achieved by an open base does not occur in a closed base. Obturation means sealing of the bore by the deformation of the projectile or part thereof. (in case the reader has not heard of that techo term before).

With the open base bullet, the force of the expanding gas softens the lead a little and also deforms the jacket a little in the process. In essence, the hot jet of high pressure gas is pushing in and expanding out the base of the bullet in an instant. The casing can only deflect as far as the grooves and then, effectively, seals. As Cinders says, there is momentary gas bypass but this is minimal for an open base. For a closed base, there is no obturation so bypass is more noticeable.

With the open base 'plugging up' the bore, the remaining the force of the expanding gas can concentrate on pushing the projectile down the barrel. IN the closed base projectile's case, there is no guarantee of a complete seal, so some gas can bypass and the possibly undersized bullet can then 'rattle' down the barrel, not completely sealing the grooves on deep/worn barrels and therefore leave the barrel in a less than intended stable spin. This leads to the dreaded 'two groove barrel' keyhole.

With worn 2 groove barrels, the owner has several options:

a. find larger .312" or greater diameter bullets to force as much seal as possible with a jacketed base.
b. switch to cast bullets and select an oversized bullet to ensure complete obturation on firing.
c. find a bullet with longer bearing surface to create a better seal, such as a 180 to 200 grain round nose.
d. the obvious - rebarrel.

My hypothesis and I could be entirely wrong.

Happy to learn otherwise.




* TLA = Three letter acronym...

I stated a specific velocity that needed to be a minimum, you probably remember my comment about shooting a hole in the pool liner back when I was doing this initial research.

By ensuring that the base velocity for Mk.7 remained above 2000, (and above 1800 for Mk.6) we arrived at a positive stabilization with no keyholes.

By checking each rifle with Milsurp ammo and a chronograph, we were able to see the velocity variation for each rifle using 2440fps as our base, then using reloads with boat tails and loading up to maintain an exceptable velocity, we were able to tailor a load to suit over 40 different SMLE's despite some of the barrels looking like sewer pipes.

So the answer to the question about some rifles keyholeing comes back to blowby caused by throat erosion. As you see, this can be overcome by specific reloading bearing in mind safety, simply by using the great variety of different burn rate powders available.
.....or you can rebarrel.

So the rifles that key hole with milsurp ammo can over come this issue by reloading and using a faster burn rate or more powder provided they use a chronny to monitor the FPS. Well it is nice to know I have been nearly right all these years except I use powders that fill the case the most. The BTs I bump out with a die, so I can with a little more work cut that process out. Bet the Boss Lady had some words about the pool liner which I don't remember.
How much variation in FPS did you notice in lot to lot or case to case using the same rifle.
Using cordite.

Throats on the lee's were generous to say the least but another malady that can present itself is cord wear unless a person knows what they are looking for at the muzzle end they may miss the fact the rifle is afflicted with such a thing and keyholing.
We are blesse3d today as companies are turning out barrels for the MkIII's both heavy and std and for the No.4's as well the problem we are faced with at the other end of the spectrum is personnel who know how to install them I mean we have BD in the US here we have TBonesmith over east I have not heard how well Beaton firearms here in W.A do up the Lee's but as you can see not huge numbers and its not just a screw in and she's apples.

There is an article in Handloader magazine I just got in the mail this week. Shows effects of subtle velocity differences on bullet expansion - fired into wet newspaper. There was only like a 50 fps difference between blunt tip and expansion - different topic. But, the pictures in the article...same bullet and rifle different velocities, might also show velocity effects on obturation. Let me see if I can find the pictures...was hoping their website had it. will need to use my camera.
EDIT: Sorry to let you down. I incorrectly thought I remembered this picture being boat-tails, and showing more of the obturation. Here it is anyway.
Attachment 113994