Fit is King - A Practical Journey

[fiver]

yeah jump is jump to touching metal.
there's other ways to center things etc.
but a jump is generally used with a slightly faster powder.
kind of like jumping a full copper bullet works best with a jump.
the cast bullet design may like some jump [or there ain't no choice like in an AR] a faster powder and a bit more strength within the alloy.

for instance.
in my 8 and 9 twist AR's.
i shoot the rcbs 0-55s bullet, i use LC cases with .0015 neck tension set with a lee squeeze die, my alloy is right about 4% tin and 6% antimony, i size to 226 [227 is as big as you can go] and dump 22.5grs of IMR-4895 in the case. [bullets are weight sorted]
now i could go a touch softer and use H-335 but i have to drop back to around 19.5grs to get the best accuracy.
maybe could do a bit better with the 335 if i could get the bullet across that nato throat and start the alignment better.

 

[Bret4207]

Hey Dimner, where ya gonna draw the line? LOL.

I already explained most of what you want to put in your fit article in the Basement Articles. It took nine articles to cover the whole SYSTEM and they are fairly brief, as in 2-3 pages each. They need to be edited and expanded considerably but I don't have the editing capability or, frankly, the time or inclination right now to do it. The gist is there.

My recommendation is to give a nod to the system (neck tension, mechanical concentricity of the case and chamber, and that sort of thing) but focus on the bullet. The simplest thing to do is define static fit and the terminology and then put a drawing and brief description of each fitment technique with notes about certain critical particulars.

Then, start this whole thing over and let's get into DYNAMIC fit as a separate topic because it literally involves everything. Put in short, dynamic fit is all about creating a good relationship between powder, alloy, and bullet design and mostly applies to fixed ammunition. Breech-seating (or muzzle seating, for that matter) takes almost all the dynamics out of the equation, which is why those methods can be so so effective to making small groups.

As I have time I can make drawings like this one for each method. Some will require two or more frames to make sense.

View attachment 27969

Nice drawing, explains much that's not easy to put in words.

 

[Bret4207]

yeah jump is jump to touching metal.
there's other ways to center things etc.
but a jump is generally used with a slightly faster powder.
kind of like jumping a full copper bullet works best with a jump.
the cast bullet design may like some jump [or there ain't no choice like in an AR] a faster powder and a bit more strength within the alloy.

Could you clear the meaning of that up please?

 

[Brad]

Could you clear the meaning of that up please?

Great example of a fiverism

 

[popper]

The mid section of Ian's drawing is what I attempt to fill. Use the front drive band, that can be sized to best fill it and provide centering (aka, Fiver's 31-168B). Angular step down to bore size, that I can't control as I don't nose size. Works for all my ARs and when I use the sized fender washer to put that step in the RD style. Cycling the AR I can see the slight scuffing of the drive band which attempts to center ( my ejector spring is really strong on that rifle). The fill question comes from some recovered pics from 303guy using stick powder. You can see large dents in the base from powder hitting the base at high fps. Implies you better keep the base inside the neck, otherwise dents in the edge of the base and bad accuracy.

 

[Maven]

Although most of the discussion is above my pay grade, here are a couple of "observations'/comments.
(1) To Ian's point that Barlow's 2 diameter (aka bore riders) CB's aren't wonderful, we can add E.H. Harrison's comment 9in the NRA publication, Cast Bullets) the .30cal. bore riding noses on some designs are indeed too small (~300") to ride the bore (should be .302"). In the same publication, the late Robert Sears showed how to determine bore rider bullet fit at the muzzle. To wit, if the CB nose falls into the muzzle up to the driving bands with little/no engraving, accuracy is likely to be OK, but not stellar*.

(2) In the early 1990's, Lyman's Ed Schmitt designed a tapered CB, #311644, for typical .30cal. ball seats/throats. It generally worked well in my .30-06, but the body cast only .3095" when .311" would have been almost perfect (I didn't know about "beagling" then).

(3) Veral Smith has been advocating something like a "pound cast" for determining mould purchases: It works! Also, driving a slug several inches into the breech and another into the muzzle and measuring them (You want the larger diameter and hope it's the one from the breech) usually results in a good cast bullet fit.

(4) While I generally like my CB's to be engraved by the rifling for best accuracy, that isn't always the case. E.g., my Marlin #336 .45-70 is nastily accurate with a variety of cast bullets seated to only 2.55". I.e., none is engraved by the rifling.


(5) Powder type and charge shouldn't be overlooked in our quest for accuracy. Sometimes an "inappropriate"/ oddball propellant shouldn't be dismissed just because it is not "standard." That's why we experiment and chronograph our loads.

(6) Lastly, some designs, e.g., SAECO #315 and its clones just work well in .30cal. rifle, including the .30-30Win, .30-06, and the 7.5 x 55 Swiss (K-31).


*The Lee Precision ~175gr. GC'd bore rider falls into the muzzle of my 8mm heavy Shaw bbl. with no engraving at all. However it is spectacularly accurate out to 50 yd. However, it loses its mojo at 100 yd.

 

[Bret4207]

The NRA "Cast Bullets" book is a good one. Thanks for tickling that memory, I need to read it again.

 

[popper]

Interesting. 168 doesn't chamber, sized 311ish - bore ring at ~310. See some scratches on the brass at the neck so large as I can go. Setting the 145 back 60 thous should work and keep the check in the neck. Should get a reasonable 2500 fps load.
Edit: the 145 sized 3085 fits. Keeps the gc in the neck. Below the neck and alloy squishes up then down to neck size, then up to throat size, then back down to bore. A lot of working the alloy before it gets to the bore. If I have some at 309/310 I'll try them else need to cast some more. Fill that empty throat space.

 

[Maven]

One more thought I had after writing the above: Accurate .22cal. (rimfire) rifles and pistols have tight (match) chambers, but the bullet of the often very expensive ammo, e.g., Eley Ten-X, isn't engraved by the rifling. I used to shoot .22 bench rest and trust me, there was no engraving of the bullet when the cartridge was chambered: Ammo quality, velocity (usually between 1,044 - 1088 fps), rifle quality, shooter skill, including the ability to read the wind made a difference, but not bullet engagement. Maybe we're exaggerating the importance of this in CF rifles?

 

[Bret4207]

One more thought I had after writing the above: Accurate .22cal. (rimfire) rifles and pistols have tight (match) chambers, but the bullet of the often very expensive ammo, e.g., Eley Ten-X, isn't engraved by the rifling. I used to shoot .22 bench rest and trust me, there was no engraving of the bullet when the cartridge was chambered: Ammo quality, velocity (usually between 1,044 - 1088 fps), rifle quality, shooter skill, including the ability to read the wind made a difference, but not bullet engagement. Maybe we're exaggerating the importance of this in CF rifles?

I have read many times that there has been more research put into the 22LR and getting it to perform well than any other cartridge. With a tight match chamber and high quality ammunition engraving of the nose shouldn't be needed. The concentricity of the loaded round in a tight chamber was mentioned by Ian and Fiver I think in the para's above somewhere.

 

[Brad]

Our biggest issue is making a round fit the chamber and throat. In ri fire competition they made the chamber fit the ammo.

Get a good barrel and a custom reamer with tight neck and a throat you like and it all becomes much easier.

 

[Ian]

Veral will be the first to tell you what folly it is to try and guide a bullet using the square inches of contact area that the tops of four skinny lands have on the bullet nose. Also, as Maven pointed out, it's good to know bore/groove at muzzle and throat end so you can determine taper. The muzzle check is a good quick one to see what won't work, but due to taper and varying amounts of wear (which we'll get into later), it really doesn't mean much because you're trying to fit the bullet to the OTHER end of the barrel. Hence the need for a chamber/throat cast of some sort.

The .22 is a heeled cartridge with a skirt and the alloy is near pure lead with some antimony in it so it will draw easily. Once the primer is lit, the bullet becomes a different shape. Dynamic fit. How good the static fit is when the cartridge is chambered determines how balanced and concentric the bullet is after its dynamic changes and thus how true it flies after exiting the muzzle. Same as centerfire, but maybe a little more dramatic due to lack of a gas check and softer bullet metal.

 

[fiver]

Could you clear the meaning of that up please?

yeah,,, sheepish look,,, ''with about .050 worth of jump'', is how the sentence should end.

you probably don't need much more than .0020 or .0030 run up with a cast bullet, but i've jumped some far enough to make a weatherby throat look close to match bolt gun tight.
usually no other option in those cases i couldn't even breech seat and fill behind with a case and a foam plug because of the throat dimensions i was dealing with. [the plug would have shot forward and jammed into the back of the bullet]
i basically worked the cases over or made them from something else to be nothing more than a tight copy of the chamber and pooked the bullet into the throat as straight and gently as possible.
there was no attempt at fitment of any sort, i just aimed the nose at the middle of the barrel and let the funnel do it's job.

 

[Bret4207]

The .22 is a heeled cartridge with a skirt and the alloy is near pure lead with some antimony in it so it will draw easily. Once the primer is lit, the bullet becomes a different shape. Dynamic fit. How good the static fit is when the cartridge is chambered determines how balanced and concentric the bullet is after its dynamic changes and thus how true it flies after exiting the muzzle. Same as centerfire, but maybe a little more dramatic due to lack of a gas check and softer bullet metal.

I think that's a biggie right there.

 

[Bret4207]

yeah,,, sheepish look,,, ''with about .050 worth of jump'', is how the sentence should end.

Ah! Now I get it. Thanks

 

[L Ross]

Veral will be the first to tell you what folly it is to try and guide a bullet using the square inches of contact area that the tops of four skinny lands have on the bullet nose. Also, as Maven pointed out, it's good to know bore/groove at muzzle and throat end so you can determine taper. The muzzle check is a good quick one to see what won't work, but due to taper and varying amounts of wear (which we'll get into later), it really doesn't mean much because you're trying to fit the bullet to the OTHER end of the barrel. Hence the need for a chamber/throat cast of some sort.

The .22 is a heeled cartridge with a skirt and the alloy is near pure lead with some antimony in it so it will draw easily. Once the primer is lit, the bullet becomes a different shape. Dynamic fit. How good the static fit is when the cartridge is chambered determines how balanced and concentric the bullet is after its dynamic changes and thus how true it flies after exiting the muzzle. Same as centerfire, but maybe a little more dramatic due to lack of a gas check and softer bullet metal.

Reference the .22. Wouldn't it make it easier if we only loaded to 1,080 fps also?

 

[Maven]

It might be easier, but <1,100 fps meant that .22 bullets were easily blown off course and even 1/16" could mean the difference between winning and losing. (You wouldn't believe the number and variety of wind flags on match days!) Btw, I never competed in the Unlimited Class (single shot, heavy bbl., fiberglass stock, expensive forward and rear rests, high magnification scope, expensive ammo) where the X was the size of the ball in a ballpoint pen. Rather, I used a heavy bbl. 7 shot detachable magazine repeater with a tuned trigger and 3 - 7 power 'scope. The X we shot at was ~the diameter of a .22 bullet and ammo such as Federal 900A or 900B or Wolf Match Extra were certainly up to the task. CCI Green Tag was their equal. but hard to find in my area.

 

[L Ross]

It might be easier, but <1,100 fps meant that .22 bullets were easily blown off course and even 1/16" could mean the difference between winning and losing. (You wouldn't believe the number and variety of wind flags on match days!) Btw, I never competed in the Unlimited Class (single shot, heavy bbl., fiberglass stock, expensive forward and rear rests, high magnification scope, expensive ammo) where the X was the size of the ball in a ballpoint pen. Rather, I used a heavy bbl. 7 shot detachable magazine repeater with a tuned trigger and 3 - 7 power 'scope. The X we shot at was ~the diameter of a .22 bullet and ammo such as Federal 900A or 900B or Wolf Match Extra were certainly up to the task. CCI Green Tag was their equal. but hard to find in my area.

Have you ever consistently shot actual 1" ten or twenty shot groups with a .22 at 100 yards? I have not been able to yet, nor ten or twenty shot centerfire cast bullet groups. 1 1/8" to 1 1/2" seems to be about my limit. I read or hear about MOA and sub MOA groups all the time. I've never seen anyone else shoot one in my presence, not have I. Five shot groups, oh yeah, ten, never, twenty seem impossible.

Since my greatest joy comes from shooting offhand I am more than satisfied for my centerfire rifles to emulate my .22's.

 

[Maven]

No experience with .22's @ 100 yd. The matches I mentioned in post #77 were always fired @ 50 yd. Btw, my heavy bbl'ed Marlin can put 5 or more fouling shots into a nickel (or smaller) group at 50 yd., but those for record had to be smaller; i.e., one (or two!) in the X and one in the 10-ring for two 10-shot targets. It's an accurate rifle, but a bit too short for off hand shooting.

 

[Bret4207]

Wind is a bear on slower moving bullets. I'm told even difference in humidity over the course of travel can have an effect. My Win 52 will shoot some small groups, but I know I'm not capable of shooting a 10 shot group under an inch at 100 yards with it even if the the rifle/ammo might be.