Calculating Freebore for a 6 Dasher

[amiabledave50]

How is it done?
I want to have a reamer made to my specific OAL. I'm using Peterson brass because of the lack of Lapua. Going to be using Burger 105 hybrid also. Trim to lenght is 1.530" with overall length of 2.441" or 1.808" using a comparator. This length allows .138" of bullet bearing surface in the neck. With these dimensions, the base of the boat tail (BT .213" long) is into the case where the body and shoulder meet. The bearing surface of .138" might be a problem down the road because of throat erosion. But I want to keep as much of the boat tail out of the case for powder.
I do not know the 105 hybrid nose taper in degrees, if that's needed. With the brass thickness being .012" I was thinking of going with a tight neck .263".

1) Is the freebore, the length of the bullet outside the case to the Ogive? (If that's the case. My freeboard is over .200")
2) is the freebore diameter .0005" larger than the bullet pretty much standard?
3) how does one figure the lead angle for a particular bullet application?
4)What is the math on how to figure out freebore? (Freebore, bore diameter, lead angle)

I've talked with a few gunsmiths. All do not have a tight neck reamer. All have different freebore lengths/lead angle. And all say that their reamer is the cat's meow.

Thank for your time.
David.

 

[rsmithsr]

is this for bench rest or prs ?? single shot or mag fed ??
i would keep the bullet base(boat tail junction) ABOVE the case neck/shoulder junction. build a dummy, and sent it to a reamer maker and have it ground to your dummy round, with the input of the reamer shop.

 

[amiabledave50]

I am not sure of the bench/prs rules. My chassis has a flat bottom with a flat bipod attachment. If I can do both I'll do both. It is magazine fed but I have Bobsled for single shot.

The boat tail bevel length measures .213". The case neck length measures .228". I don't think .015 grip will hold the bullet. If that's what you're asking.. .. That's why I have .138" of grip. Which would mean the boat tail is further into the case.

What would you say is the proper grip for a 6mm bullet in the neck? When I'm available to get to my records. I'll have to check my 6 PPC. I'm not sure offhand what it is.

 

[rsmithsr]

no that is not what i said...or thought i said. the boat tail does not count, the junction of the boat tail and the main dia of the bullet should stay above the junction of the case shoulder and the neck.
with your numbers , case of 1.530 and the shoulder at 1.309..means your neck is about .221...
if you use 170 of the neck for seating ( not counting the boat tail), the bullet bt/body junction is about .050 away from the neck

 

[amiabledave50]

no that is not what i said...or thought i said. the boat tail does not count, the junction of the boat tail and the main dia of the bullet should stay above the junction of the case shoulder and the neck.
with your numbers , case of 1.530 and the shoulder at 1.309..means your neck is about .221...
if you use 170 of the neck for seating ( not counting the boat tail), the bullet bt/body junction is about .050 away from the neck

Thank you for your response. I'm going to take you up on what you said and I'm sending a dummy round, empty case and bullet to Manson reamers. Talked with Ray there. One hell of a nice guy. And didn't mind answering a bunch of questions. I see where your figures came in. I can say now that I cut the the neck lenght to .200". Case still 1.530". There's a .060 radius at the base of the neck and didn't want to cut into that fear of weakening the cartridge.
I magic markered the bullet when inside the case. I then removed the bullet. It also showed a bearing surface in the case of .138". I also talked with Burger bullets. I wanted to know why they had an OAL dimension of 1.256" when I'm getting 1.282". They said all the dimensions on their site very lot by lot. Even the base to Ogive! Doesn't make much sense to me being that the base to Ogive dimension is critical. So my figuring was on Burgers dimensions. I can say that the BT and bearing surface are closed to what they have. I do not have an optical comparator. On my Ogive comparator I get a base to Ogive of . 604 and not their. 624. The gentleman from Burger said they made three 6 mm comparators and they all read the ogive differently.. .