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Design questions - Mountain Molds

Design questions

highstandard40
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Joined: Tue Nov 06, 2012 9:25 am

Design questions

Postby highstandard40 » Tue Nov 06, 2012 10:00 am

I'm a past customer of yours from your home state. You cut a 360-255 gr mold for me a while back. It has proven to be very accurate and is a real deer thumper in my 358 Win.

I have an XP100 chambered in 7TCU that I use in IHMSA silhouette competition. This gun has a very long throat. Up to this point, I have been unable to find a commercial mold that makes a bullet that performs well in this chamber. All my accuracy issues can be blamed on bullet fit. I have all the dimensions of the chamber and throat and would like to design a bullet to best fit this chamber. I have a couple of questions I hope you can answer.

1. I want a bullet of 160 grains that I can seat with the base seated flush with the bottom of the case neck, that fills the long throat, and has a bore ride nose. I can design that with your software. With a single lube groove located in the neck, I end up with a very long front drive band of full .287" body diameter to fill that long chamber throat. In your opinion, would I be well served to add a lube groove to this area of the bullet. It would be exposed when seated, I'm OK with that if it's needed. I was just wondering here if a .330" long section of the bullet body would be OK with no lube or does it need a groove.

2. Since the idea of IHMSA is long range accuracy and retained energy, I need a bullet with as slippery a shape as I can get. I can sacrifice some coefficient for good fit and accuracy though. Your software suggests a meplat no less than 63% of bullet diameter but will draw bullets with less than that. Is a smaller meplat even possible with your equipment, or does it have to be at least 63%?

I look forward to your response. Thanks

Charlie B.............

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mtngun
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Re: Design questions

Postby mtngun » Tue Nov 06, 2012 3:20 pm

I'm afraid I can't cut smaller than 63% meplat in 7mm.

I would be leery of a 0.330" long front band. It's not about the lube, it's about bullet distortion. Grooves allow the bands to be engraved easily with minimum distortion.

A cast bullet needs to be supported at 2 places when it chambers: 1) the nose should lightly engrave the rifling, and 2) the cartridge neck should be a reasonably snug fit in the chamber. What happens in between is not that important.

I'm not a fan of the "glove fit" theory of making the bullet fit the entire throat. I've never seen any advantage to it, and sometimes have seen poor accuracy, probably due to excessive bullet distortion.

Just my 2 cents.

highstandard40
Posts: 6
Joined: Tue Nov 06, 2012 9:25 am

Re: Design questions

Postby highstandard40 » Tue Nov 06, 2012 5:16 pm

OK, thanks for the input. I may be looking at this from the wrong perspective. I have tried the RCBS 145 and 168. Neither are a bore rider and I have to seat them deep into the case. Accuracy was very bad, especially with the 168. I have an NOE 150 gr mold that is a bore rider but it's only .955" in overall length. If I seat this bullet to the base of the neck, only .180" of the bore rider part of the nose is into the rifling, but it is not engraving because of the tapered leade. I can seat the bullet out more, using only half the neck and it still does not engrave but accuracy is better. The throat in this chamber is .287" in diameter. In your opinion, should I size to .286" or .287" to get good rear support for the bullet and then go with a longer bullet to reach the rifling? These are my chamber diminsions.
Neck----.270" long
throat-----.287" in diameter and .320" long

Your thoughts?

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mtngun
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Re: Design questions

Postby mtngun » Wed Nov 07, 2012 3:12 pm

There are many variables.

-- one simple rule of thumb, a good starting point, is to size the bullet 0.002" over groove diameter and the nose 0.001" over bore diameter. Seat out until the nose engraves slightly.

-- another rule of thumb is to size the body of the bullet to result in 0.002" - 0.004" clearance at the chamber neck. Any tighter than 0.002" clearance will likely result in difficult chambering. Sometimes this neck clearance is the one that matters, other times the neck clearance can be very sloppy and the gun will still shoot well. Note I said "neck," not "throat" or "freebore."

-- it's a toss-up which bullet design will shoot best in a long freebore. The temptation is to fill up that freebore with a fat front band (or bands), but the benefits of a snug fit may be offset by increased distortion when the oversize front band engraves. Also, as a practical matter, you'll need 0.002" clearance in the freebore to ensure reliable chambering. A 0.287" X 0.320" front band in a 0.287" X 0.320" freebore will be asking for chambering problems. More likely the front band would either have to be 0.285" for reliable chambering, or else use a much shorter front band.

-- at the end of the day, the only way to determine the optimal bullet diameter is to experiment with different diameters. If you order a 0.287" bullet, you can always size it down to 0.285" or so if it shoots better at that size.

Re: the RCBS 168. the photo at the RCBS website shows what appears to be a bore rider. Ditto their photo of the 145. What do the noses of your bullets actually measure ?
'Image

What does the nose of your NOE bullet measure ?

How much clearance does the cartridge have in the chamber neck when a 0.284" bullet is loaded ?

What is your bore (not groove) diameter ?

I've shot 1000's of cast in the 7TCU, deep seated with bullet lube exposed to the powder space, and accuracy was generally 2 MOA - 4 MOA, depending on the load, in a 10" TC with standard sights. The load that I settled on for IHMSA was 2.5 MOA, if I remember correctly. But that barrel had a pretty ordinary throat, not super long freebore like yours. I was shooting a 170 gr. NEI bore rider designed for the Mauser, but the nose was way too fat to be a slip fit in a TC, so I just seated it deep. That fat-nose bullet shoots sub MOA in a 7/30 Water TC carbine, and it actually prefers to be seated deep so that it has to jump to the rifling.

highstandard40
Posts: 6
Joined: Tue Nov 06, 2012 9:25 am

Re: Design questions

Postby highstandard40 » Wed Nov 07, 2012 6:04 pm

I have an RCBS 145 mold and at one time had the 168 also. Both had a nose that measured .2805".,.....Both had body diameters of about .2845" when using COWW + 2% Tin. Both would not "lightly engrave" the nose when chambered. I could not close the bolt on a round with the bullet seated so that the nose entered the rifling.The 145 shot 3-4MOA and the 168 was terrible........12" groups at 100 yards. I have sized to .285" for all test lots with every bullet so far, RCBS and NOE. The RCBS of course a little smaller than .285" Basically just lubed them. All were lubed with "Felix Lube"


1. What does the nose of your NOE bullet measure ?

2. How much clearance does the cartridge have in the chamber neck when a 0.284" bullet is loaded ?

3. What is your bore (not groove) diameter ?


1. The nose on the NOE 150 is .2765"........it is a slightly loose slip fit in the muzzle of my XP. I would guess I could go to .277" and be OK.
2. I don't know about case neck clearance but I have recently loaded cartridges with bullets sized .277" and they chamber without any undue effort.
3. I have slugged the bore and whereas I can assure the groove is .284" I don't know how to accurately measure the bore. I don't have any pin gauges.

I have a new group of cartridges loaded to test and here is an issue with testing for me. There is no local range near me. I have a place to shoot at my family's rural property which is 200 miles from here. I plan to travel there Friday for some hunting and will do some range testing while there. As I mentioned, so far I have been sizing all my bullets to .285".......This new group will have bullets of .287" in COWW +2% Tin and also some loaded with linotype/COWW mixed 50/50. Both of these are seated out as far as I can get them.

To date, my best groups have been with the lino/WW alloy and AA2015 powder. I ladder loaded with SR4759 and 2015. 2015 groups were less than half the size of the 4759 loads and the 2015 groups got smaller as the powder charge increased till I got to 24.5 gr and then the groups stabalized somewhat at around 2.25" at 100 yards. These 5 shot groups were fired from sandbags. My hope is to get down to at least 1.5 MOA.........1 MOA would be better. Is this realistic? The gun is a proven performer. With Jacketed bullets I won 2nd place in International class at the 2008 IHMSA International Championship with this gun and I won this year's Regional Championship, so the gun will shoot. At least with jacketed. I had also scrubbed the bore clean before I started the cast bullet testing.

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mtngun
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Re: Design questions

Postby mtngun » Wed Nov 07, 2012 9:18 pm

My ISP-from-hell has ate my response twice so I will try again in the morning. :lol:

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mtngun
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Re: Design questions

Postby mtngun » Thu Nov 08, 2012 10:08 am


highstandard40
Posts: 6
Joined: Tue Nov 06, 2012 9:25 am

Re: Design questions

Postby highstandard40 » Thu Nov 08, 2012 10:30 am

Thanks for the input. For now I'll continue "tweaking" and see where it leads. I'll try to decide on specs for a new mold if indicated and get back to you.

highstandard40
Posts: 6
Joined: Tue Nov 06, 2012 9:25 am

Re: Design questions

Postby highstandard40 » Wed Nov 21, 2012 10:47 am

Update.

This past week I tested some loads with the NOE bullet sized .287" and seated out as far as possible. The bullet nose still does not contact the rifling but the chamber throat is filled to the point that closing the bolt is a little tight. 100 yard, 5 shot groups are now about 1.125".......much better. I'm thinking, based on our conversations and range results, I'll start designing a mold and aim for sizing to .286" to assure trouble free chambering, have a long bearing surface, then a bore rider nose of sufficient length to engage the rifling. Weight will likely end up 160-165 grains. Thanks for all the advice. I'll be in touch

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mtngun
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Re: Design questions

Postby mtngun » Wed Nov 21, 2012 1:55 pm

Glad you're making progress ! :)


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