Print Page | Close Window

distortion on bullets

Printed From: Enfield-Rifles.com
Category: Enfields
Forum Name: Enfield Gunsmithing
Forum Description: Submit any how-to's or other gunsmithing suggestions here.
URL: http://www.enfield-rifles.com/forum_posts.asp?TID=6690
Printed Date: September 20 2020 at 8:56pm
Software Version: Web Wiz Forums 12.01 - http://www.webwizforums.com


Topic: distortion on bullets
Posted By: hoadie
Subject: distortion on bullets
Date Posted: March 01 2014 at 11:44pm
Now I ask this cuz I don't know..
I was layin in bed last night thinking about the statements some folks on the forum have made about how their various Enfields don't seem to like certain types of bullets. So I got thinking..Is there a possibility that the actual bullet base distorts upon being fired?
ie: if a cartridge has a flat based bullet, does that bullet have any distortion when the powder explodes? Of course my musket - using Minnie balls does, but that's different.
Could the various QUALITY of these said bullets, be suspect as to why some Enfields don't shoot proper?
Just asking..the more I think about it, the more it seems logical to me.(Course, I will no-doubt be "re-educated" shortly)
I await the wisdom of the Great Oracle!
Hoadie

-------------
Loose wimmen tightened here



Replies:
Posted By: paddyofurniture
Date Posted: March 01 2014 at 11:57pm
You are one smart cookie Hoadie and a deep thinker.


-------------
Always looking for military manuals, Dodge M37 items,books on Berlin Germany, old atlases ( before 1946) , military maps of Scotland. English and Canadian gun parts.


Posted By: Shamu
Date Posted: March 02 2014 at 12:26am
You rang?


I'm not sure. The originals were stepped with a reduced diameter boat tail, but I don't know of any modern bullet with that shape.

The only thing that comes to mind is variable bore diameter, if the bore is .314instead of the "theoretical" .311 I can see the shorter bearing surface maybe having less "grip" to spin up the bullet. Probably have the same effect withan under diameter bullet like the .3105" Hornady's for the Moisin Nagants as well.


-------------
Don't shoot till you see the whites of their thighs. (Unofficial motto of the Royal Air Force)


Posted By: hoadie
Date Posted: March 02 2014 at 1:04am
AHA!! So I DO have a point, after all.
See, I'm not just another pretty face
Hoadie

-------------
Loose wimmen tightened here


Posted By: Long branch
Date Posted: March 02 2014 at 9:21am
Those "stepped" bullets are called Rebated Boat Tail bullets. They tend to have higher ballistic coefficients than regular boat tails. The flat surface around the tail prevents gases from being funneled around the bullet, theoretically offering greater consistency in velocities. I don't know of any company that makes such a bullet for the 303. Lapua makes them in .308. I'd love to try some in an enfield with a 2-groove barrel. Guess I'll have to save up for a swager.

When the rifling bites into a bullet, metal gets displaced. More bearing surface means more gets displaced. It seems to me that this would fill in the gap in an oversized bore. That would explain why some enfields only like round nosed bullets. The old military flat-based bullets had the lead exposed. It may be that the pressure wave pushes the soft lead core, expanding the bullet a bit like a minie ball.


Posted By: muffett.2008
Date Posted: March 02 2014 at 5:11pm
 Yes hoadie, the rear of the projectile does distort, or obturate, being the correct terminology.
 The flat based 303 projectiles were open base, this allowed the "bump" from the cordite to spread the copper walls as it pushed the projectile down the barrel, giving a reasonable seal in often eroded sections of leed in, at the same time the swaging of the outer jacket being pushed to the rear, ensured a tight seal to prevent bleed bye as the projectile  pushed forward.

 This obturation also occurs to a lesser extent in boat tails, the military Mk.7z and Mk.8, being relatively hard skinned, created early problems in eroded throats by allowing excessive bleed before the projectile began to swage, sometimes causing keyholeing, due to lower velocity.

 Modern boat tail projectiles have a softer skin and longer bearing surface to alleviate this problem, only the diehards that don't reload keep the old stories alive about boat tails being no good in 303's.


Posted By: hoadie
Date Posted: March 02 2014 at 7:34pm
AHA! Another "oracle"!
So, laying awake at night aint a waste of time after all.
There's only one question about this remaining in my mind.
Is there a noteable QUALITY difference between manufatureres with regards to the actual projectile?
We certainly see a difference in powders, & "over the counter" ammo.
Hoadie

-------------
Loose wimmen tightened here


Posted By: Shamu
Date Posted: March 02 2014 at 9:26pm
"only the diehards that don't reload keep the old stories alive about boat tails being no good in 303's."

I'm  not sure I'm  understanding that, so excuse me if my answer is a bit "orf".

I don't believe ALL Enfields don't like BT's I've had 2 that liked them just fine. However I've had a savage with a 2-groove bore that absolutely hated them. Bore diameter was normal if you slugged it, but that wouldn't reveal an eroded throat. This gun would make actual 90 degree sideways keyholes at 15 yards with both Hornady & Sierra BT bullets while shooting FB's perfectly.

Based on this I guess the correct statement would be:
"Some 2-groove bores will not fire boat tail bullets well, but some will."


-------------
Don't shoot till you see the whites of their thighs. (Unofficial motto of the Royal Air Force)


Posted By: Long branch
Date Posted: March 03 2014 at 10:03am
Originally posted by hoadie hoadie wrote:


There's only one question about this remaining in my mind.
Is there a noteable QUALITY difference between manufatureres with regards to the actual projectile?
We certainly see a difference in powders, & "over the counter" ammo.
Hoadie


I'm working up a load with the .312 hornady BTHP bullets and reloader 15 at the moment. I've noticed that the meplats are more uniform than those of the sierra BTHPs. They are doing well for me so far. The diameters are consistent. If they have any variance, it's too small for my digital caliper to read.


Posted By: muffett.2008
Date Posted: March 03 2014 at 5:12pm
Shamu, the only reason the odd No.4 does not like boat tails is because of excessive throat erosion.

 If you were to chronograph the rifle with the two different types, you would find a marked loss of velocity compared to a rifle with good barrel.

 This is why I say if you have keyholing, increase the load to see if it stabilises, 90% of the time it will.

 You can do a chamber cast to check throat erosion, or use a target grade borescope or endoscope.


Posted By: muffett.2008
Date Posted: March 03 2014 at 5:29pm
Hoadie, once again yes, the profile changes with boattails also.
 The privi 174 milspec pill is as per the original Mk.8z, so it lacks the longer bearing surface that the Sierra Matchking and Hornady have, although its jacket is soft, which aids in sealing.


Posted By: hoadie
Date Posted: March 03 2014 at 7:54pm
Well, now I know.
Not only have I learned more..I've actually instigated some discussion! (local boy does good.
Many thanks for the input and sharing!
Hoadie

-------------
Loose wimmen tightened here


Posted By: Shamu
Date Posted: March 03 2014 at 8:52pm
I'm curious. How does throat erosion correlate to 2-groove bores which the problem seems confined to?


-------------
Don't shoot till you see the whites of their thighs. (Unofficial motto of the Royal Air Force)


Posted By: Long branch
Date Posted: March 04 2014 at 1:45am
I thing the problem with the two groove barrels is the way they're done. On the savage-made guns, they just have two small grooves. The "lands" are huge. They depend on the bullet swaging out into the grooves. The grooves on on the one I had were deep. This would create an opportunity for gases to bleed. As has been said, increasing the load may solve the problem. I don't know of anyone that has experimented with it.


Posted By: muffett.2008
Date Posted: March 04 2014 at 5:26pm
 Actually the problem is across the board, it may well be more noticable in two groove barrels there because of their proliferation in your part of the world.
 Longbranch touched on one of the reasons, another being that the chambers in most North American No.4's were slightly larger, as commented on in previous posts where I gave the dimensions of the chambers of rifles from 1900 to 1945, the last few being No.4's.
 A slightly larger chamber and increased freebore, mean a millisecond delay between the ignition and the first contact with the lands, before pressure can build to expand the case effectively enough to seal the chamber, a very black neck shows a degree of backbleed, gas escaping around the projectile as it begins to move forward and superheating the throat surface as it is forced past the bullet in it's forward travel, increasing the erosion of the metal both at the throat and the leeds.
 Remember, the cartridge case is a sealed container, with a restricted ignition point and a point of low resistance to ensure that the pressure pushes one way instead of exploding, the lands simple offer a slight point of resistance to ensure pressure is maintained sufficiently long enough to allow the swaged projectile to exit the tube.
 Adding a twist to them simply creates the spin to stabilize the projectile in it's flightand flattening it's trajectory due to maintained velocity.
As opposed to a round ball with high parobolic trajectory and lower range/velocity.
 The bleed buy effectively reduces the velocity to a point where the projectile cannot stabilize rapidly enough to prevent erratic flight.
 By chronographing each firearm, and using the Milsurp standard as a velocity guide, we are able to see the velocity drop in the odd firearm compared to one that holds a velocity equal to what the milsurp ammunition produces, along with a velocity drop of several hundred fps, we can also note the changes in the bullet strike and grouping as the firearm heats up.
 So assuming we reload to 2440fps, +/- 50fps, and we suddenly find a particular rifle is only giving about 2000 fps(about what the old black powder round produced) we can safely assume that we have a severe bleed problem.
 Checking those cases for pressure signs should show none, other than a stretched base if the pressure allows the case to move rearward before it grips the sides of the chamber, with this in mind we can start to gradually increase the load until we either see pressure signs, or the projectile ceases to gyrate/keyhole and we achieve a more realistic muzzle velocity.
 This is a reasonably safe commonsense procedure, remembering that pressures are measured under controlled conditions in tight chambers and unworn barrels, pressure signs on a brass case will always be noticed long before the actual safety paremeters are exceeded, by using a slow burning powder as opposed to a faster one, you ensure that the risk of overflash and case capacity cannot be exceeded.........however you will find that the problem will be solved with only a slight increase in the load required, it really is only a fine line between good and mediocre. 


Posted By: Zed
Date Posted: March 04 2014 at 8:32pm
Very interesting topic and useful information. Thanks for sharing the knowledgeThumbs Up

-------------
It's nice to be important, but it's more important to be nice!



Print Page | Close Window

Forum Software by Web Wiz Forums® version 12.01 - http://www.webwizforums.com
Copyright ©2001-2018 Web Wiz Ltd. - https://www.webwiz.net