3 degree throat angle and bullet selection

[MS50]

I'm in the process of rechambering a 308 to 6mmbr. The barrel I intend to use is 1:8, 26 inch, and has a .1163 freebore and a 3 degree throat angle. I understand that with this freebore I will be able to reload 100 class bullets with no issues. I've searched and haven't found specific information on what the 3 degree throat angle means to me. I want to start acquiring components for reloads. Does the 3 degree angle favor certain lengths and/or shapes or bullet? Thanks for any feedback or directions to a resource that will shed light on this.

 

[michaelbz]

I believe most people use 1.5 deg per side. Does your reamer drawing say 3 deg inclusive or per side? I have not found much on leade except for the following documents. If you do not want to read them, they found 2.4 deg leade superior to 1.2 deg. I welcome other opinions as well

http://www.dtic.mil/dtic/tr/fulltext/u2/a431357.pdf
http://www.dtic.mil/ndia/2009infantrysmallarms/tuesdaysessioniiisiewert.pdf

 

[MagnumManiac]

The throat angle has little to no bearing on bullet shape requirements, the angle at the start of the rifling does, some are better suited to secant ogives than tangent ogives, but most don't worry about this.
Does your chamber have a straight leade, then a tapered 3º throat, or does it just taper from the case mouth clearance cut?
A tapered throat, generally, is more accurare if it has a straight leade, then the taper, but this is not always the case. The 300 Win Mag does very well with it's 3º tapered throat without a straight leade.
The tighter the throat, say .0015"-.0005" taper , is more conducive to accuracy than larger tolerances.
I wouldn't be concerned with the tapered throat, just pick a bullet and test it.

Cheers.
;D

 

[MS50]

My drawing reading skills are limited. It looks like 3 degrees per side. You're articles are informative. The secant vs tangent graphics are great. What started me down this road was reading that bullet design, for example hybrids, can affect load development results, depending on throat taper. I'll use these loads for medium to long range steel. Loading flat based bullets for short range use will come later. The leade appears to be straight, with the 3 degree throat. This barrel is a no-turn neck, which is what I want. I'll pick a tangent ogive bullet and go. I've also read that it doesn't take long for a 3 degree taper to become a 1.5 degree taper. In effect, I'll be chasing the lands within a few hundred rounds. Thanks for the feedback bz and MM.

 

[Steve Blair]

The throat angle has little to no bearing on bullet shape requirements, the angle at the start of the rifling does, some are better suited to secant ogives than tangent ogives, but most don't worry about this.
Does your chamber have a straight leade, then a tapered 3º throat, or does it just taper from the case mouth clearance cut?
A tapered throat, generally, is more accurare if it has a straight leade, then the taper, but this is not always the case. The 300 Win Mag does very well with it's 3º tapered throat without a straight leade.
The tighter the throat, say .0015"-.0005" taper , is more conducive to accuracy than larger tolerances.
I wouldn't be concerned with the tapered throat, just pick a bullet and test it.

Cheers.
;D

[br]
I assume you refer to included angle 3°, as is European practice. American prints, like the attached SAAMI chamber drawing, refer to angle relative to bore axis. Same thing, expressed differently. The reason .300 Win has zero freebore is SAAMI OAL. .300 Win Mag target chambers, like the one I shot in the eighties, have long freebores because of the relatively short neck.

 

[spclark]

3 degrees per side(or 6 degrees total for the design) I believe is well beyond what any competent reamer designer would specify. Lead angle this abrupt would radically increase pressures.

There are several popular reamer designs for a variety of cartridges that feature a 1-1/2 degree leade angle (3 total) so I'd suspect that's what you're dealing with. You should be fine with any popular 100 - 108 grain bullets.

 

[MS50]

OK 3 degrees total or 1.5 per side makes sense. I tried to figure out what the B=basic was in the print legend but did not. I hoped that would shed light on the question. I didn't doubt that the reamer is competent. I just wanted to get it straight in my head what I was dealing with.

 

[michaelbz]

If you want to use 3 deg per side, go for it. I just wanted to make you aware not many people do. A spec 260 rem reamer is actually 3 per side, so it is safe. Some use .5 deg, too. There is very little data either way, when that happens most choose to follow the herd. I personally would use a well recommended gunsmith, tell him the caliber and bullet, and let him choose the reamer and barrel blank. If you get a rifle that doesn't shoot or has other problems, you have only one entity to deal with and you avoid finger pointing.

 

[expiper]

OP,,,,"B"enchrest shooter found that you can make it anything you like,,but,,after 200+ rds,,,it wears to ~1.5 degrees no matter what it was to start,,,Roger

 

[ireload2]

If a drawing complies with the US national standard ASME Y14.5M "basic" means a dimension with NO tolerance.
This means that it is
1. A gauging dimension used with very close tolerance gauges such as the shoulder headspace circle diameter.
2. Or it is a dimension that is determined by other geometry on the drawing.
The lead angle is really determined by the change in diameter and the length of the lead. So who cares about the exact angle?
The lead angle is really a truncated cone. It is easier to measure the beginning diameter (big end of the cone), the ending diameter (small end of the cone) and the length of the cone. Then you can use trigonometry to calculate the angle.

The information above can be found in this standard but it does take some study to understand it.

https://en.wikipedia.org/wiki/Basic_dimension

OK 3 degrees total or 1.5 per side makes sense. I tried to figure out what the B=basic was in the print legend but did not. I hoped that would shed light on the question. I didn't doubt that the reamer is competent. I just wanted to get it straight in my head what I was dealing with.

 

[expiper]

the idea of having a ramp there is to allow the bullet jacket at the ojive to ramp or glide into the lead and not rip or damage the jacket and cause unnecessary bullet failures,,,1.5 deg per side is about right and is how the bore seems to wear with the type/style bullets in use today,,,(spitzer/vld/etc)..Roger

 

[MS50]

The barrel I intend to order is a pre-fit McGowen. I'm going to go with it and wear it in, apparently to a 1.5 degree from the available information. I've only heard good things about these barrels. Thanks for the feedback. I've learned a lot about the subject over the last few days.