Gas blow-by on BT bullets.
- Thread starter hoofpower
- Start date
The bullet has engaged the lands and is already deforming to the internal groove and land dimensions before the shank has left the neck
Steff
Well-Known Member
The issue with boat tail bullets isn't gas blow-by within the barrel. It's about the kick in the ar.. it gets when it leaves the muzzle. I'm no expert in this and I am lacking the vocabulary to describe it correctly, so please be gentle on me.
What happens when a bullet exits the muzzle is that it is actually overtaken by the gases following it down the barrel. These gases will form a whirl and impact the bullet at the base. If both the gas whirl and the alignment of the bullet are not 100% 'square' there is a chance that the sloped part of a BT bullet will receive an asymmetrical kick to one side.
This is why flat base bullets are considered to be more precise for short to medium distances. Only on the long range (>300 m) will the ballistic advantages of a BT bullet outweigh this crook.
What happens when a bullet exits the muzzle is that it is actually overtaken by the gases following it down the barrel. These gases will form a whirl and impact the bullet at the base. If both the gas whirl and the alignment of the bullet are not 100% 'square' there is a chance that the sloped part of a BT bullet will receive an asymmetrical kick to one side.
This is why flat base bullets are considered to be more precise for short to medium distances. Only on the long range (>300 m) will the ballistic advantages of a BT bullet outweigh this crook.
I would have thought that the reduced surface area of the base of a boat tailed bullet may reduce the effect of gas overtaking the bullets base. My understanding of the flat base bullets being preferable for short range was that the increased turbulence in the wake of the flat based bullet brought about stable flight earlier than with a boat tail. I recall some Lapua boat tail designs are stepped to harness this effect, ie cause increased turbulence. Of course I may be way off the mark so am happy to be corrected
I thought the ballistic advantage of BT bullets at longer range cf. shorter range was because was because of the improved BC resulting from the BT shape, rather than any disadvantage from the BT shape at shorter range - though srvet's point about the bullet 'going to sleep' quicker with a flat base is interesting.
The evenness of the gases passing the bullet after it leaves the muzzle is AFAIK a property of the crown. The bullet's base which the gases are passing will be revolving at 180,000rpm - so minor unevenness there will be averaged out fairly well, I'd imagine.
The evenness of the gases passing the bullet after it leaves the muzzle is AFAIK a property of the crown. The bullet's base which the gases are passing will be revolving at 180,000rpm - so minor unevenness there will be averaged out fairly well, I'd imagine.
Brilliant thank you.....
Although spinning very fast, the bullet is only making one turn in the length of your rifle's twist. E.g it will have to have travelled 12", 10", 8" or whatever, from the muzzle, to have made just one rotation. So no, I don't think that it averages out.
The muzzle blast does overtake the bullet initially, but it's soon left behind.
If you use a moderator or muzzle brake, that could also greatly affect what happens in those first few inches from the muzzle. What gas flow goes on inside a moderator, whether that might buffet the bullet, I can only speculate.
The crown is all-important. The perfection of the bullet base too, equally so I'd guess, flat bases are supposed to be less difficult to make consistent than boat tails, but I suspect that the differences with good bullets aren't huge. And it could rather depend on the gas pressure behind it just as the bullet leaves the muzzle, which is largely a function of powder choice, loading, and burn characteristics.
The good news is that whilst all bullets leave the muzzle slightly cock-eyed, even from supposedly perfect match barrels with match chambers, 11 degree target crowns, and trued straight actions, they do stabilise ("go to sleep"), assuming that they are well made and suited to the velocity and twist. That may take some distance though.
My limited experience with .223 was that flat bases were the most accurate to say 300 yards, beyond that the boat tail designs started to show their advantages.
Once they are in the air, the perfection of the pointed end is all-important. . This is where the shock wave starts, any imperfection here has the potential to send them off-course. The shock wave moves back down the bullet onto the ogive as it slows, even towards the tail, but whilst it remains highly supersonic the effect at the tail is I suspect less of a factor, though still important. It's the overall shape that matters, boat tails do help here, the better ones are shaped to minimise "drag", i.e. the energy dissipated in creating the shock wave. Subsonic or trans-sonic designs, well that's a different subject.
Here's some Schlieren photography demonstrating this:
Flat bases generally carry their mass to the rear, boat tails can help distribute it further forwards, which is supposed to be better at longer range. You can also over-stabilise a bullet if you spin it faster than necessary, then it won't stay in line with it's trajectory, becoming increasingly angled up as range increases, with increasing drag and other bad effects.
Hornady make quite some claims for their ELD-X bullets, take a look at Heat Shield® Technology - Hornady Manufacturing, Inc for example.
I can't remember where I saw it, but there was an article where the points of psp bullets were deliberately damaged to see what effect that had on POI/accuracy - and the as far as I can remember, the effect was surprisingly limited.
Damage at the base of the bullet, however, made a considerable difference.
Does that experiment/article sound familliar to anyone?
Maybe this one ?
The Box O' Truth #49 - Damaged Bullet Accuracy and...
I have had friends that were deer hunters that would closely inspect their hunting bullets and discard any that showed any damage whatsoever to the point of the bullet. Is this a good idea, or does it even matter? Let's find out. First, I ordered a box of hunting bullets with an exposed lead...
www.theboxotruth.com
Massively damaged soft-point bullets that would have been rejected by anyone sensible, and filing a groove into the base hardly representative of real-life, though it did of course send them flying off. Nothing really learned from this exercise. At just 100 yards I think. Minute of pie-plate, no better.
Hunting bullets of whatever construction, at our hunting ranges seem to be mostly OK. Step up your game and extend the range, then precision starts to matter. Start target shooting over longer ranges and the differences quickly become apparent. There have been improvements over the years, and it would be silly to entirely reject them, hype notwithstanding.
Hornady's white paper about this stuff, subtly criticising everything else, with selected photos to make their point, but, what else should you expect:
Or just shoot round balls.
Steff
Well-Known Member
I have recently read on a bullet manufacturer's homepage, cannot remember which one, that they refuse to add polymer tips to their lathe turned monolith bullets. The reason being that it would add an imperfection to the tip of an otherwise perfectly symmetrical bullet.
Looking at some of the polymer tipped bullets with the plastic nipple lovelessly crushed into the front cavity I have full appreciation for this standpoint.
Looking at some of the polymer tipped bullets with the plastic nipple lovelessly crushed into the front cavity I have full appreciation for this standpoint.
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