- This topic is empty.
-
Topic
-
Anonymous
I have this mold, 249-39-RF-E6 2 Cavity RG2 PB Brass, and so far it drops true to spec at .249″ (.250″ where the mold closes), but I really need at least .2505″ to shoot accurately. I haven’t shot the slugs yet, but I do have some “advertised” .249″ NSA slugs (actual measure is closer to .250″) that group like bird shot, and the advertised .250″ (closer to .251″) NSA slugs can make one ragged hole. Ideally I would like the slugs to drop as cast at .252″ or larger and size down to .251″. Sure, I should have gotten a different mold, but this mold is shaped exactly like the .250″ NSA slugs that are like a laser out of my gun, so I want it to work.
What is the “best” compound to use for lapping/bulging the holes? I’ve read on some other forums that folks like to use Comet, baking soda, or even tooth paste (basically all the same thing) to lap molds, but I don’t know if that is aggressive enough to remove .002 or .003″ of material. I have all three on hand, but I’ve also read folks using products like Flitz to lap molds.
I also don’t mind if the slugs are a bit lighter, and I’ve read that adding some tin to my melt could help “fatten” the slugs. Is there an ideal ratio? Normally I cast in pure lead, but I’ve been adding recovered H&N, Crosman, and my own pure lead air rifle pellets/slugs to my melt lately.
While I do have powder coat and a spare oven, I’m trying to save myself from having to PC the slugs every time, and I plan on shooting a lot of these.
-
Replies
-
Mike B.
One might possibly suggest getting a harder honing material … which is preferable for softer materials like brass.
Pure lead (as cast) is going to tend to shrink…
I also tend to believe that I have read that some of the airgun users posting that they do tend to add a tiny bit of tin to the mix (~ 1% ) to get a slightly larger slug.
Also adding a bit of tin to the mix to make a honing blank might help, as it will tend to help fill out the mould to get you a tiny bit closer to the actual cavity size. You might just end up gaining a half a thousandths more with a tiny bit of tin, & thus solve your accuracy problem without taking a chance to mess up the mould with uneven lapping with the materials that you originally suggested.
There are also companies that specifically manufacture and offer honing materials & honing oil for this purpose …
Honing is a science all by its self… The internet tends to list specific honing materials appropriate for brass under honing materials … Softer metals like brass tend to need a harder honing materials …
Lastly there are machinists that are setup to regularly do things like this … including opening up the OD bands on moulds, like your slug… However that can potentially be as expensive as the mould itself… I have had this done my self… and have been happy with the mould modifications.
Your call on how you want to proceed.
Take Care, & Stay Safe…
One might possibly suggest getting a harder honing material … which is preferable for softer materials like brass.
Pure lead (as cast) is going to tend to shrink…
I also tend to believe that I have read that some of the airgun users posting that they do tend to add a tiny bit of tin to the mix (~ 1% ) to get a slightly larger slug.
Also adding a bit of tin to the mix to make a honing blank might help, as it will tend to help fill out the mould to get you a tiny bit closer to the actual cavity size. You might just end up gaining a half a thousandths more with a tiny bit of tin, & thus solve your accuracy problem without taking a chance to mess up the mould with uneven lapping with the materials that you originally suggested.
There are also companies that specifically manufacture and offer honing materials & honing oil for this purpose …
Honing is a science all by its self… The internet tends to list specific honing materials appropriate for brass under honing materials … Softer metals like brass tend to need a harder honing materials …
Lastly there are machinists that are setup to regularly do things like this … including opening up the OD bands on moulds, like your slug… However that can potentially be as expensive as the mould itself… I have had this done my self… and have been happy with the mould modifications.
Your call on how you want to proceed.
Take Care, & Stay Safe…
I readily admit that I would rather not hone the mold myself. I have zero precision tools. Adding tin to the mix sounds much more appealing to me as a first try. I wouldn’t mind a slightly lighter slug either, reducing weight by 10-15% would be even more desirable and get closer to my “ideal” velocity of 1050 FPS (950-1050 is my target range). Is there a table or formula to know how much tin to add to reach a target weight? I know there are tables to show the increase in BHN hardness, but what about weight?
I might just have to experiment, which could take a while.
The formula I’m trying to figure out is: X% lead + Y% Tin = 85-90% of weight compared to pure lead. (36.5 grain vs 32-ish grain)
I have a program that calculates the alloy mix… including the SPG.
I also setup an excel program to convert the calculated alloy SPC to final cast bullet weight… as a double check scenario…
The math is there on the internet … it just takes a bit of programing to set up the program to input the data in the proper spots in the formula.
Finally, I also basically have the same software that is used to design the cast product that NOE uses…
The version that I have also contains an option to change the alloy in that program… once the dimensions for the cast product are entered … that changes the predicted final cast bullet weight…
You many have noticed that the Pellets & Slugs are run with pure lead, while the smokeless product is normally shown with wheelweights… Changing the alloy selection also changes the final predicted weight.
PM me if you are interested in a prediction, & I will run some numbers for you.
Take Care, & Stay Safe…
OBTW changing the predicted alloy from pure lead to a 30:1 alloy equals a predicted weight of ~38.3 grains.
One doesn’t get to ~ 36 grains until one casts with Linotype alloy
Mike B.- You must be logged in to reply to this topic.