question about boat tails vs flat based keyholing

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Scotty
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question about boat tails vs flat based keyholing

Post by Scotty » Sun Mar 18, 2018 3:00 pm

I have a rifle that was custom built for my father 75 years ago. It is a 243 and deadly with any flat based bullets I have tried. However, it will NOT handle boat tail bullets and key holes them all over the place. I do not know the twist of the barrel. I don't know how long BT bullets have been around, however I suspect that this barrel was made before they existed. Anyone have any experience with this same problem, The barrel is a Douglas supreme, the action a Mauser from a 1903 Springfield. My father shot out 2 barrels of 30-06, and had it re-barreled in 243 win. This was done around 50 years ago. Makes a great varmint and target gun. Just have to be careful when cycling a cartridge since it is a long action. I never use more than one in the chamber and one in the mag. Not a gun for a kid.

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marlinman93
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Re: question about boat tails vs flat based keyholing

Post by marlinman93 » Mon Mar 19, 2018 9:45 am

Without a lot more info, I can't tell you why whatever boat tail you're using is keyholing? Would need to know all the things you mentioned you don't know in order too determine why. I can say that in my bolt action high power rifles the boat tail bullets perform better than flat base, and I always try to use them if the weight, diameter, and shape I need are available in a boat tail design.
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Re: question about boat tails vs flat based keyholing

Post by Scotty » Mon Mar 19, 2018 12:07 pm

I also have a pre 64 Mod 70 in 243 win, and it shoots both BT and FB bullets equally well. However, the Custom rifle we are discussing will out shoot the 70. It just will not handle a BT bullet. Guess I will have to stop in and see my gunsmith. It has to be the twist, I believe.

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marlinman93
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Re: question about boat tails vs flat based keyholing

Post by marlinman93 » Tue Mar 20, 2018 9:29 am

As I mentioned, without knowing the twist rate of the barrel, and the weight of the bullets, I couldn't begin to give any input. Kinda like asking "why is my car so gutless?", but not knowing anything but the cubic inches of the engine.
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Re: question about boat tails vs flat based keyholing

Post by Brent » Tue Mar 20, 2018 4:02 pm

I shot a lot of lead boattails and I found they are terribly unstable. That wasn't news, but the point is simply if you are shooting similar length boattails as plain base, the latter will be stable flying from a lot of rifles that the boattails fly AOT. You need a much faster twist to keep them point on.

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Re: question about boat tails vs flat based keyholing

Post by Scotty » Thu Mar 22, 2018 6:53 am

The bullets that I used in the above mentioned 243 with the key holing problems were Sierra 85 gr and 100 gr flat spitzers and 85 and 100 gr game kings, Shoots both spitzers perfect, and throws the BT'. game kings. Both loads are 38 gr of 4064.

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Re: question about boat tails vs flat based keyholing

Post by marlinman93 » Thu Mar 22, 2018 10:09 am

Have you measured the two styles of bullets with a good micrometer to compare diameters? Could be the boat tails aren't correctly sized to your bore. I've learned not to trust what the boxes say for diameters until I check a few.
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Re: question about boat tails vs flat based keyholing

Post by Scotty » Thu Mar 22, 2018 10:29 am

Thanks for that info. No, I have never checked the diameter of the two bullets. Now that this subject is on my mind, I remember my father who died 38 years ago telling me that "this rifle won't shoot just anything". Fact is, I didn't pay that comment much mind since he was doing all the reloading, and I just did some of the shooting. Obviously, he had run into the same thing. Does anyone know how long BT bullets have been common and popular. Another thing, if the diameter of the bullets are different, why would they both shoot so well in the mod 70 winchester. That thing will shoot anything you put into it. That is the rifle I hunt with and have it sighted in for 100 yds. Doesn't matter what you put in it, it will group.

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Re: question about boat tails vs flat based keyholing

Post by Brent » Fri Mar 23, 2018 6:00 am

Twist rate is everything to a boattail. It needs to spin very fast, for a given bullet length) because it has no (or at least much less )base drag to keep it straight. The relationship of bullet length (not weight or diameter) to barrel twist is what matters.

If flat based bullets shoot so much better, why bother with boattails at all? It seems you have the solution to your problem well in in hand.

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Re: question about boat tails vs flat based keyholing

Post by Scotty » Fri Mar 23, 2018 8:04 am

Brent and others. Of course you are correct. I really don't have a problem, and I do use only FB bullets in the rifle we are talking about. I just wondered WHY this problem exists. I never talked to anyone who ever mentioned this same problem. I am also not educated in the difference that twist makes when that bullet is traveling down the barrel. I will try to improve on that. You guys are great and I appreciate the help. Thank you all.

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Re: question about boat tails vs flat based keyholing

Post by marlinman93 » Fri Mar 23, 2018 8:49 am

Twist rate has a large effect on accuracy, and the weight and length of a bullet's accuracy is dependent on it's relationship to twist rate and velocity. Sometimes if a bullet is pushed faster or slower it can compensate for the twist rate being a little off. What works for a flat base bullet isn't necessarily going to work for a boat tail bullet.
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Re: question about boat tails vs flat based keyholing

Post by Brent » Fri Mar 23, 2018 9:16 am

Scotty wrote:
Fri Mar 23, 2018 8:04 am
Brent and others. Of course you are correct. I really don't have a problem, and I do use only FB bullets in the rifle we are talking about. I just wondered WHY this problem exists. I never talked to anyone who ever mentioned this same problem. I am also not educated in the difference that twist makes when that bullet is traveling down the barrel. I will try to improve on that. You guys are great and I appreciate the help. Thank you all.
Scotty,
The best I can explain it is the following
1. as every bullet flies, there is pressure on its nose to slow down. The perceived wind on the nose, tries to slow the nose down without having that effect on the base so the back end of the bullet, wanting to faster will attempt to pass the front end of the bullet. what stops this from happening is anythign that creates drag on the back of the bullet to keep it safely behind the nose. things like grease grooves on cast lead bullets might help to some extent, and fletching like an arrow would work, if only we could figure out how to keep the darn feathers on.

But what does happen with flat based bullets is that a vacuum is created behind the bullet. Low pressure caused by the displaced air that has not yet rushed back to fill the void that the bullet just vacated. That has the same effect as a drag chute on a top-fuel drag race car.

In the case of the boattail, there is much less of this vacuum because the boatail occupies what would normally be the highest part of the vacuum. So, less base drag means less drag overall for the bullet, which is good, but there is less of a hold on the base of the bullet keeping it behind the nose where it belongs so it tries to pass the bullet nose and now you have instability, really bad aerodynamics on the nose part of the bullet, and, in worst case scenarios, tumbling.

The solution to this lack of base drag in a boattail is to spin it faster. Like the tops that children play with, the faster you spin it, the harder it is to tip over. There is gyroscopic stability in place of a drag on the back of the top that keeps it upright. And that is how one keeps boattails point on. Spin them faster.

You can get faster spins (in terms of rotations per second) by launching them at a faster velocities, but there are obvious limitations to how much you can do this before you blow up a barrel. So, faster twist barrels (rotations per inch) will give faster rotations per second, with a little more safety.

Perhaps that makes some sense as a back of the envelope description of what is happening with boattails. They lack that suck on their backsides to keep them straight that flatbased bullets have.

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Re: question about boat tails vs flat based keyholing

Post by Scotty » Fri Mar 23, 2018 3:55 pm

Wow Brent. I don't need to search any farther. I will have to re-read your post a few times to absorb all the info you included. I really appreciate the info. What started this for me is : I wanted to get my 14 year old grandson some experience, so I wanted him to load some shells and then go to the range and shoot HIS reloads. So, I took out a few game kings and showed him how to load them, and he loaded them all. Off to the range we go. The gun was already sighted in at 200 yds. We were shooting at steel targets. So, I sat down beside him, and he (who is a good shot) fired off a rest and I saw the dirt fly about 4 feet from the target. I asked him what target he was shooting at and he told me. Shot again a couple feet the other side. I took the rifle with the same results. My first thought was the scope, Checked everything for loose, and all was good. A couple more shots with the same results. Another shooter asked me if I had any factory rounds and yes I did, so went home and got some factories and they shot perfect. That is when I confirmed that it won't shoot the boats. To say the least, it wasn't my finest hour with my grandson. Thanks for all your help.

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Re: question about boat tails vs flat based keyholing

Post by Brent » Fri Mar 23, 2018 5:24 pm

That is certainly not my best piece of writing, but if you can make sense of it, you should get an intuitive feel for what's going on. This is dramatically simplified and there are other things that happen, but I think this is the gist of what you are dealing with.

When you are driving down the interstate, you might notice that some semi trailers have panels that fold out past the rear doors, then are angled backwards and if you use your imagination, you can see them as being, effectively, a boattail like your bullet. Same principles are in play, though there is no rotational option of course. Anyway, they are trying to reduce the "suck" behind the truck to improve mileage. Daytona stock cars draft each other for much the same reason as well, the first car benefitting from having the second car fill that vacuum off its rear end.

Sounds to me like you both had a very successful day at the range - you guys learned lots.

Happy shooting,
Brent

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