Thoughts on throat and lead design

[tomme boy]

I am going to have a new barrel installed on one of my guns here shortly. I am trying to figure out this whole idea on the throat and leade for the barrel. Parallel or a tapered throat or the leade angles are what I am looking at. I know there are some really smart people here and I am looking for help. I have some understanding of this but the more I can understand, the better.

[GabbyM]

What caliber bore?

[tomme boy]

0.308"x0.300" 308 win. I have had a long discussion with Goodsteel about doing the work. I have settled on a barrel. And a cartridge. Now I just need to figure out the throat and leade.

One thing I want to add. This is going to be for target shooting. I don't care about running it fast so don't go there. I don't want to hear it. I want to know about which way to make it accurate.

[MBTcustom]

My thought is that the throat of the rifle should have no freebore. The angle of the rifling and how it attacks the boolit (best description I can think of) is a subject I have thought about a lot.

I know that rifles with very obtuse lead angles have trouble shooting cast lead sometimes, especially when you start pushing the envelope, and I figure that the folks who designed the 30-06 had it right. 1 degree 22 minutes per side is what they went with. I figure that for our purposes, a little shallower would be better, but I don't like the Mauser design of 25 minutes 26 seconds. I feel that it would promote boolit skid and slump, but I freely admit my mind is not 100% made up on that. I keep watching to see if anybody has had any luck with the Weatherby magnums as they also have a very very long throat to reduce bolt thrust.

My philosophy is to make the boolit fit the throat and lead angle like a glove, (I have enjoyed reasonable success with this method) but that requires a relatively obtuse throat angle. I recommend 2 degrees 53 minutes included, or 1 degree 25.5 minutes per side.

Like they say, you pays your money and you takes your choice.
Even the really obtuse throats that are only about 1/16" long (like are found on old marlins) are able to be made to shoot insanely well. They love paper patch boolits.

[runfiverun]

the 30-06 throat works just fine, they pretty much copied it for the 308.
I'd go for the tapered over a long parallel.

oh and fast and accurate go hand in hand what get's you one can get you the other and vise versa.

[tomme boy]

I was under the understanding that a very short freebore was bad for cast. I had a talk with Al at NOE the other day and he said he used a 308 win print to design the bullet I sent you Tim. He tried to run the exact angle of the design on the CAD program he had and it came up at 2*. He said it had to be with the way the program rounded the #'s as to why he could not find a better #.

So just looking around it looks like the 308 win has a 1.5* leade. And as I was looking around everyone was saying to use a 1.5* or less down to most BR guys are using a 0.5* angle. But this was also for using jacketed bullets. They are going to be a secondary use for me. I use 95% cast. But sometimes I use the jacketed just because.

[williamwaco]

0.308"x0.300" 308 win. I have had a long discussion with Goodsteel about doing the work. I have settled on a barrel. And a cartridge. Now I just need to figure out the throat and leade.

One thing I want to add. This is going to be for target shooting. I don't care about running it fast so don't go there. I don't want to hear it. I want to know about which way to make it accurate.

I used to be a software developer. I learned one very important thing in 30 years. The worst software on the planet is the software that was developed by the user telling the development team how to do it. The best software occurs when the user tells the development team what they want the software to do and then gets out of the way.

That paragraph is ALL you should tell him - and then get out of the way.

[MBTcustom]

I was under the understanding that a very short freebore was bad for cast. I had a talk with Al at NOE the other day and he said he used a 308 win print to design the bullet I sent you Tim. He tried to run the exact angle of the design on the CAD program he had and it came up at 2*. He said it had to be with the way the program rounded the #'s as to why he could not find a better #.

So just looking around it looks like the 308 win has a 1.5* leade. And as I was looking around everyone was saying to use a 1.5* or less down to most BR guys are using a 0.5* angle. But this was also for using jacketed bullets. They are going to be a secondary use for me. I use 95% cast. But sometimes I use the jacketed just because.

I have to agree with R5Rs comment. fast and accurate go hand in hand. You can hold back and use the principles of FIT to give you nice tidy little bugholes, or you can use it to ramp up the speed. Both are hard to do, and both require the same techniques.

So past a certain point, (I would call it 1/5 degrees) FIT is the name of the game. FIT is King. Doesn't matter what your lead angle is, you are going to require it to damage your boolits in an controlled, concentric manner. There are several ways to accomplish this.
1. the Lovern style.
This style ignores your lead angle and instead just uses a whole bunch of easily engraved shallow bands. For this reason it's very versatile, and does pretty well in most situations because it just becomes what it needs to be as it is engraved.
2. The Morse taper fit style
This style is the one I subscribe to, but it requires some fairly specific parameters to be met by the lead angle/length. The idea is to solve perfect engraving by being, well, perfect! as soon as the boolit moves .010 forward it is engraved over 65% of it's length. This is the method I used in the XCB project.
3. The bore rider style.
This boolit attempts to solve perfect engraving by guiding the nose loosely withe the inner surfaces of the lands and using the base section to spin up the front. This boolit design is prone to slump.
4. The hourglass style.
This style is a bit of a hi-bred between the Morse taper and the bore rider. This is the style you sent me with your rifle barrel (whether it was intended to be designed as such or not). Basically, the widest part of the ogive sits on the lands like a bore rider, sways in the middle and then contacts the lead again on the angle that just protrudes from the casemouth. This boolit hesitates to engrave just a little bit, and then swarms the rifling suddenly. Almost like a hollowground knife edge. The MiHeck 30 SIL is of this style.

Each of these basic rifle boolit styles have been used to produce exceptional groups but each have their own strengths and weaknesses.

[tomme boy]

Thanks Tim. I am hoping others chime in. No one has ever explained the type if fit like that that you just wrote about. That is what I am looking for. An explanation of the types and why.

I was looking over the 30 SIL booilt and was trying to figure out why it is accurate for people. It looks like it has about a 45% bearing surface. Or less. But if it works, why does it work. This is what I want to know. If we can do this with out the arguing that seems to always happen it would help all on this board to understand a little bit more about what is going on once that trigger is pulled.

[MBTcustom]

The reason people argue is because noone is able to sit on the boolit and ride it down the barrel, so different people have different theories about what happens when the boolit is engraved.
I chose the MTF style because even though everyone seems to agree that it's the right way to go (from many many discussions on FIT over the years) hardly anyone has actually followed that method to the hilt with a boolit design. I did, and it's surpassing all my expectations.

The hi-bred hourglass shape like the 30 SIL has been reported to give very good accuracy and I have to admit that as far as fit goes, when seated in the throat, I got little to no wiggly movement. It was trapped snug. However, I haven't personally been able to get good accuracy out of it compared to the "one size fits all" lovern design, or the MTF design.

I actually like the Lovern style and truth be told, I've sent more of those down range than any other boolit style. (even though at the time, I was doing several things totally wrong in my load process.) It just covers a multidude of sins.
My problem with it, is that it often requires seating below the neck by quite a bit, and the standard torpedo shape could be more aerodynamic.

[Quiettime]

My thought is that the throat of the rifle should have no freebore...

Why is that? Because you don't want the boolit to slam the rifling after it's already moving?

[DougGuy]

Well, I have a 1970s production M77 in .308 that I did a lot of work on. I hogged out the stock, carefully steel bedded the entire action all the way to the forend, pillar bedded the action screws, lapped the locking lugs, ended up tying a 100rd box of loaded .45 Colt to the front sling swivel so it would pull down on the stock, took this off after the bedding set up and now I have an upward force the length of the barrel. It likes this upward force very much.

Before I did the bedding, I free floated the barrel, omg wrong thing to do! Groups doubled. That's how I knew to put upward pressure on it.

When I was worknig up loads in fireformed brass, the very last thing I did was seat a dummy round long and push it into the rifling and close the bolt on it. I opened the bolt, very gently pushed the dummy round out and set it in the press, cranked the seating die down on it until it stopped. Removed the dummy and turned the seater 1/2 turn and locked it in. It now has what .020" or .025" freejump before it engages the rifling? This, was the final tweak that cut groups from 3" at 200yds to putting 3 into a guitar pick at 200yds!

The cannelure is about .100" in front of the case mouth so it had that much freebore in the throat. Also, I might add that it has a 1:10 twist, that does NOT like boattails, and will shoot to less than 1/4 MOA on a cold barrel, first shot, with a 180gr plain base over 41.5gr H4895.

Hit a turkey from 340yds once with this rig, I was aiming 1" below his beard, the boolit hit him 1 3/8" below the beard, and 1/4" off centerline. Not bad to hit within 1/2" of point of aim at 340yds!

I'm sure goodsteel knows full well what he is doing with a throating reamer, this is just my experience with mine, tossed in for reference..

[MBTcustom]

Why is that? Because you don't want the boolit to slam the rifling after it's already moving?

There's nothing inherently wrong with freebore if you fit it. The hi-bred hourglass style do well for this, as do the Lovern style. The bore riders have a hard time, and the MTF style kinda look sideways at it with a puzzled expression. All in all, I just don't see what inherent advantage the freebore has. It doesn't really hurt, but it doesn't help either. As the boolit engraves, I don't want any part of that boolit to slide into a parallel walled tube (freebore) that's a waste of time. I want it to be engraving and always getting smaller as it heads for the crown, and I want the throat to start working on it as soon as it leaves the case neck. I wouldn't have a problem with it, if the freebore was tapered so that it was bigger at the case mouth than it was at the lead, but then it begs the question, why does this tapered section not have rifling in it?
I could be wrong but I see it as wasted motion. When its all said and done, the most important thing is that the boolit actually fits whatever lies in the 1/2" past the casemouth.