Rifle Rounds: Your Acceptable COL Tolerance; Die Tolerance Expectations

[otisrush]

Caliber: .243
Bullet: Hornady 105gr HPBT Match
Dies: RCBS - standard 2 die set.
Purpose: Load Development Testing
Gun: Remington 700

My loading up until this gun has been semi-auto rifle and pistol for plinking. This is the first caliber/gun I'm trying to get "moderately serious" about building some loads that really perform well out of my gun. (And I've been REALLY happy with the results so far.)

My COL goal for these loads was 2.750".

First question: When folks say they want (for example) a COL of 2.750" - what tolerances do you accept as "close enough"? 2.748-2.752? Narrower? Wider? "None of the above! I go for 2.750" with each cartridge!"? Tested COL at Hodgdon's site was 2.76". So I wasn't letting anything through below 2.750"....but I had a few 2.751s.

Second question: What sort of tolerance should I expect when I lock down the seating plug? I was getting frustrated trying to get it locked down. I ended up leaving it loose and after each round I'd back out the plug 1/4-1/3 turn so with each round I'd incrementally bring down the plug until I got to my COL. Prior to doing that I was finding that 1/ when I'd tighten the locking nut I'd screw in the plug more no matter how hard I tried to keep it stationary and 2/ I'd find some cartridges that would end up "considerably" shorter COL - such as 2.744.

If I'm overthinking this - fine. But I'm curious to hear how other people approach these two different items. (How close is close enough? And what sort of COL variance do you expect from cartridge to cartridge if/when the seating plug is locked down? When it was locked down I didn't seem to be getting consistent COLs - they'd vary by .003"-.005".)

Thanks.

OR

 

[hdwhit]

You shouldn't be able to go wrong adjusting the dies in accord with the instructions RCBS provided with the dies.

I don't load .243, but I do use RCBS dies for .223. What I did with it was to:

• Loosen the locking nut
• Back the seater plug out
• Run a round into the die and progressively screw the seater plug back in until the desired COL was reached.
• Using a screwdriver to hold the seater plug in place,
• Tighten the locking nut using a pair of pliers.

I set up my .223 seating die this way in the early 1980s and until recently when I started loading some FMJ bullets with a cannellure, had not changed the setting in more than 30 years and it was still producing rounds with a COL that didn't vary by more than 1/1000 inch.

Remember that even for bullets of the same weight, the shape of the bullet's nose (called an "ogive") may differ from one supplier to the next and this can affect seating depth and COL. Be particularly aware of this if you bought bullets from a company that sells "de-militarized", "pulled" or "factory second" bullets since what you receive may include bullets from more than one manufacturer.

 

[ReloaderFred]

Most rifle seating dies use a seating stem that contacts the ogive of the bullet, not the tip of the nose. This is because hunting bullets for the most part have exposed lead tips and lead will deform when in that small a diameter. If you measure the length of your bullets from the same box, you'll probably find some variation among them, and it may be that variation that you're actually seeing.

People get pretty hung up on Over All Length (OAL), but unless you're loading right up to the leade in the chamber, what is really important is the volume of the loaded round, which has a direct bearing on pressure. Most people load anywhere from .010" to .020" off the lands for rifles. Some load closer and some load farther away, but there's usually enough space there to account for a small variation in the OAL.

Hope this helps.

Fred

 

[Berger.Fan222]

Adjusting the die is really changing the distance from the spot on the ogive where the die is pushing the bullet to the back end of the cartridge. COAL is the distance from the back end of the cartridge to the bullet tip.

Some bullet designs vary a lot more between where the die contacts them and the tip, leading to greater COAL variations. My experience is that plastic tipped bullets have less variation than open tipped match bullets and lead tipped spitzer style bullets.

The practical issues for a variation in COALs are feeding from a magazine, accuracy, and whether the bullet ends up in the rifling when the bolt is closed. If one of these areas is problematic, have a closer look. If not, figure your variation is mostly in the front end of the bullet, and stop worrying about it.

 

[Bart B.]

A 243's case head is not against the bolt face when fired but it's shoulder is hard against the chamber shoulder at that time, so it's the distance from case shoulder to bullet diameter at the rifling diameter that determines how far the bullet moves to the rifling. The .242" diameter on the bullet is about where it first touches the rifling. There's a few thousandths spread in case head to shoulder so that is the spread in bolt face to case head when fired. Bullet tips touch nothing when chambered and fired.

There are small tolerances in bullet shapes, a few thousandths inch spread in length dimensions is normal. Besides, 243 chamber throats erode away about .001" down the bore for every 20 shots fired. Match grade 243 barrels have a 1500 round barrel life and accuracy degrades 1/4 MOA at short range for a given cartridge OAL. Your rifle will last twice that long before any significant accuracy loss can be seen.

An OAL spread of .010" has been observed on some commercial match ammo that's very accurate. That much on your reloads won't matter.

A more critical dimension is case headspace; head to shoulder dimension. It shouldn't have a spread of more than .002" for best accuracy. Full length sizing fired cases to set their shoulders back .002" for bolt guns is about right. Gauges can be made or bought to measure it.

 

[kingmt]

I never measure. I keep a standard in with the dies that I pop into the shell holder & adjust the die to it.

 

[z7]

I have noticed hpbt bullets have slightly different lengths due to the tip being slightly different each bullet. If the ogive is consistent then each seated bullet should have rhe same or very close base to ogive measurement.

If you are adjusting the die each bullet you are adjusting the "jump" to the lands. As bart pointed out it may not matter if you have a good load. For my part i am ok with slight oal variations as long as my ogive is consistently placed.
I would not adjust the die each time, find a length that works, load and shoot

 

[otisrush]

Thanks all. Some great info has been provided.

The thing I'm trying to figure out is why the COL would vary from cartridge to cartridge even when the seating stem position doesn't change. With my other calibers I set the stem and COLs are consistent. Period. But in this situation being discussed in this thread the resulting cartridges will sometimes have some COL variance.

One thought I had: As you can see these bullets I'm using are L O N G. I'm now guessing the bullet is contacting the seating plug prior to the bullet being completely engaged in the die neck. Therefore, variance from true vertical bullet alignment (relative to the case) as the bullet/case enter the die might change where on the bullet the stem is actually engaging the bullet....meaning the bullet would/could be cockeyed. If one bullet is more cockeyed than another it seems it could result in different COLs. (The seating plug would contact the bullet higher on the bullet the more cockeyed the bullet is.)

I know for sure the tip is not engaging the top of the plug, as I get some very faint markings on the bullet from the plug, part way down from the tip.

OR

P.S.: I'm asking this because it's bugging me that I can't figure out what is going on. The resulting loads are performing GREAT. So this situation is not being pursued as a result of having bad performance at the range. It's just bugging me and I keep asking myself "Why?".

 

[30Cal]

Because the points of the bullets aren't identical. The plug contacts below the point because that's the way to get jump to be more consistent. Jump is slightly important; COL is not important.

 

[Bart B.]

Otis, it's impossible for bullet forming dies to shape a few thousand bullets from as many round copper disks so their tip is always the same distance from any other part of the bullet across all of them. Search the web for "bullet making pictures" and you'll be amazed at how complex bullet making really is. It starts with a copper disk the size of a coin, then it's cupped and drawn a few times, trimmed, stuffed with a lead core then pointed. At 1 per second or faster.

People making the best ammo possible don't fret over a few thousandths cartridge or bullet length spread. Nor should you. Can you cut carrots to exactly the same length?

There are bullet hollow tip (meplat) trimmers you can buy that'll trim them to the same length with less than a .001" spread.

I see no difference in accuracy through 600 yards as bullet jump to rifling increases almost 1/10 inch from barrel wear.

A more important measurement is making sure the bullet diameter is larger than the barrels groove diameter by at least .0003". .0020" larger is fine. Anything smaller, accuracy gets worse. That's why the best match grade barrels' groove diameter is smaller than the best match bullets' diameter.

 

[otisrush]

I'm mentally satisfied given the info people have shared. Like I said, I wasn't worried, more curious.

Thanks all!

OR

 

[243winxb]

Measure COL off the bullet ogive, not the tip. Use a comparator

 

[Walkalong]

Or use a good fitting seater stem and don't worry about it at all. But I guess we need to use that comparator to see if it is to start with.

That said, I don't worry about it. Use a good seater and good bullets and worry about shooting, not OAL. Just make sure it is reasonable and there isn't something wrong.

 

[witchhunter]

Good info here, I agree with the COL length being "just a number used to set up your die" once the die is set, the differences will be caused by variance in the ogives of the bullets you are using. Remember if you change bullets to another brand or style, you will have to readjust your die. I love Redding micrometer dies just for load workups. They are repeatable and it makes it easy to adjust .005 up or down. You have ten loads at varying depths, when you get back from the range you can load the one that worked by using the micrometer. It is just easier. You can do the same thing with a standard RCBS 2 die set, it is just a lot more work measuring and adjusting your seater die. I did it for years, now I have a micrometer seater for almost all of my bolt actions.

 

[Bart B.]

Please!

When measuring anything from the bullets' ogives, the only place that matters is its diameter where it touches the rifling. That's the bullet diameter .006" to .008" less than bullet diameter. Any smaller diameter is meaningless; it touches nothing in the barrel. Except to ensure they'll load from a magazine.

This should be mentioned. Twist the bullet's pointy end into the rifling at the muzzle; that's the small diameter ring scratched on the bullet; a good starting place.

 

[Walkalong]

I would assume that is where the comparators are made to contact the bullet. I dunno though, don't use them.

 

[MrSpiffy]

The main reason I pay close attention to COAL is because I'm loading for my AR-15, and my max is 2.260" so it can fit in the magazine. With heavy bullets, you load to max length for the magazine, not for the distance from ogive to rifling. For that reason, I shoot for 2.257" to 2.260" Sometimes the powder compresses a little differently and I get one shorter than that, but if it's longer I just adjust the die down a tad and get it to 2.260"

For a bolt action rifle, I'd measure a little differently and have a +/- tolerance. Maybe +/- 0.002"..?

 

[Bart B.]

What happens to bullet jump distance to the rifling if the COAL spread is .001" but the spread in case head to shoulder is .003"?

 

[GooseGestapo]

Besides what fits magazines, some actions like lever actions with tubular magazines require yet shorter oal to function.
i.e: Marlin .338MX has maximum operating length at 2.585". 1893 Marlin at 2.495". Longer will feed, but not eject loaded cartridge.

 

[otisrush]

I have to say.....although I knew of bullet comparators previously I wasn't precisely clear on their purpose.

Now I know!

 

[Bartojc]

What happens to bullet jump distance to the rifling if the COAL spread is .001" but the spread in case head to shoulder is .003"?

Is this directed at the OP or whomever wants to answer ? I'll give it a whirl to see if it makes sense

If ones spread in case head to shoulder swings .003 + or - you would need to add or subtract from the COAL depending which way you were over or under your desired measurement. Assuming you are using a comparator to measure COAL, if you were loading to the lands it could be bad causing pressure to spike if you were on the wrong side of the +/- . Case in point if your case head to shoulder were under your desired measurement by .003 and your OAL was over by .001 that's .004 closer that the bullet will be to the lands. If you load right on the lands that may be enough. In other words it can be more than just one measurement that has an effect. Is that what you are getting at ?
I love your questions Bart, it makes me think.

-Jeff

 

[Sunray]

"...the COL would vary from cartridge to cartridge even when..." Assuming all the locking bits of the die are locked tight, it's operator failure. Means you're not operating the ram exactly the same every time. Could also be a bit of give in the bench itself. That gets compensated for by you. However, a thou or 2 one way or the other isn't going matter much. SAAMI max OAL is 2.710" and that's from the pointy part to the flat part. No ogives involved. No idea why Hodgdon chose to go over SAAMI spec. Accurate does too though.

 

[otisrush]

Is this directed at the OP or whomever wants to answer ? I'll give it a whirl to see if it makes sense

If ones spread in case head to shoulder swings .003 + or - you would need to add or subtract from the COAL depending which way you were over or under your desired measurement. Assuming you are using a comparator to measure COAL, if you were loading to the lands it could be bad causing pressure to spike if you were on the wrong side of the +/- . Case in point if your case head to shoulder were under your desired measurement by .003 and your OAL was over by .001 that's .004 closer that the bullet will be to the lands. If you load right on the lands that may be enough. In other words it can be more than just one measurement that has an effect. Is that what you are getting at ?
I love your questions Bart, it makes me think.

-Jeff

That's what I figured Bart was getting at - if he did indeed direct that question to me. I interpreted it, maybe incorrectly, as more of a rhetorical question (which is why I didn't respond) to have folks think more holistically rather than be over-focused on just one dimension.

It's an excellent point.

OR

 

[Nature Boy]

The thing I'm trying to figure out is why the COL would vary from cartridge to cartridge even when the seating stem position doesn't change

Neck tension variation = OAL variation.......at least that's what I've found

 

[Walkalong]

Powder compression can equal OAL variation above normal. I haven't seen it in neck tension variation that I noticed. Maybe I just didn't notice. I would think it would have to be with very tight vs light neck tension, which we shouldn't have if we are doing it right.