Freebore, COAL, and chambering specs a miss!

[Buelrdr29]

Good day all! I had a custom barrel put onto my Sako A7. I dont want to throw any companies u der the bus until they have had a chance to respond to my service request (submitted to their web customer service already).

Here is the question. I had a barrel made for me and a prefit chambering for 7mm rem mag. The barrel went on smoothly and was checked with gauges. All work was by a very qualified gunsmith. He mentioned the length to the rifling was long. I had been having accuracy issues with several factory loads and my own loads. So i took the gun back to him and he measured 3 different bullets with the hornady modified case tool. What we found was a COAL from 3.535 ro 3.685 to touch the rifling. All were 175 gr bullets from differents makes. Now the 7mm rem mag has a saami spec of 3.29 as max and anything longer does not fit my mag.

So the two questions I have: is this an obvious cause for accuracy inconveniences, basically a .245 jump? And is this way out of spec for a new barrel? It will only get worse as the bore erodes. What would you expect from the barrel maker?

 

[gbflyer]

If you load some long and single feed them does the accuracy live up to your expectations? That’s the only way to tell for sure if it’s an issue or not.

I have a very long throated 30-338 and it shoots anything at any length into a hole that just gets bigger. It’s a mile out to the lands.

 

[Buelrdr29]

Have not tried going past mag length. There is no way I am single feeding out in the mountains either hunting or target shooting. I may try a few after hunting season.

I did have a 3 shot group that was .30 inches at 100y. But then a lot more that were up to 2 inches. It is just so inconsistent. None of my other rifles have been this way. And I have tried several handloads with different powders and bullets. As well as factory stuff.

 

[BrBa]

Does your barrel have an appropriate twist for the 175 gr bullets? I kinda doubt the jump is ruining your accuracy.

Just out of curiosity have you measured the approximate coal if you were to seat the bullet with the base of bearing surface at the shoulder/neck junction? It probably won't fit in your magazine, but it would be interesting to see how much jump that would be to the lands if you had to load each round by hand. There might be remedies available with a different bottom metal that will accept those 3.715" aics mags.

6.5x55 military rifles have very long throats as well, and yet people seem to get very good accuracy with everything from 120 gr to 156 gr bullets in those old small ring mausers.

 

[Buelrdr29]

The twist is 8.5 rather than a normal 9.5 it came with. Specifically talked with customer service when I ordered it and told them I wanted to shoot the 175 gr ELDX. So twist rate is good. Also verified with the barnes twist rate calculator.

 

[wyosam]

They may have chambered it for that bullet seated with the base to the neck/shoulder junction. If that wasn’t requested, then they should have cut the chamber to standard specs. Chambering in a faster twist, with a long bullet, I can see where they might have made that leap, but they should have clarified what your mag length limits are.


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[Buelrdr29]

They said they only use a saami spec reamer and they had dummy rounds thr reamer was built too. Said it was 3.310 COAL. Mag fits 3.29 which is saami COAL.

 

[BBob]

Now the 7mm rem mag has a saami spec of 3.29 as max and anything longer does not fit my mag.

SAAMI max OAL length has no bearing on if a bullet will touch lands at that length or shorter. Some SAAMI throats are way out there, some are short and some in the middle. Just because SAAMI states a max OAL does not mean bullets will touch lands anywhere near max OAL. Did you provide dummy rounds with your choice of bullets and request it to be throat-ed to those? No experience with SAAMI 7 Mag so someone else will have to comment on that particular throat length.

 

[Buelrdr29]

SAAMI max OAL length has no bearing on if a bullet will touch lands at that length or shorter. Some SAAMI throats are way out there, some are short and some in the middle. Just because SAAMI states a max OAL does not mean bullets will touch lands anywhere near max OAL. Did you provide dummy rounds with your choice of bullets and request it to be throat-ed to those? No experience with SAAMI 7 Mag so someone else will have to comment on that particular throat length.

All good perspective. I did not provide bullets or dummy rounds. There was no option even on the reamer. Just a standard chamber cut. I agree that they are not guaranteed to touch. But a .569 jump seems huge to me for a standard barnes 175 gr tsx. And .245 jump for a hornady eldx 175 gr.

Just seems excessively long.

 

[BBob]

They said they only use a saami spec reamer and they had dummy rounds thr reamer was built too. Said it was 3.310 COAL. Mag fits 3.29 which is saami COAL.

It sounds as though the reamer body is SAAMI but the throat may not be. I'd ask what the freebore length is and and what angle. For reference the SAAMI 7mag freebore is .1142" @ 3°. That sounds short until you factor in the 3° angle. The effective seating depth on a 3° compared to the common aftermarket preference of 1.5° is much longer with 3°.

Just seems excessively long.

I'd tend to agree. I'd ask the above questions about their throat specs and see where you are.

 

[TaperPin]

told them I wanted to shoot the 175 gr ELDX.

I may be wrong, but what you said can be interpreted two ways. One way is just for twist rate, and the second is with custom chambers they might have set the freebore to match the bullet when it’s full diameter base is even with the start of the neck, which is not what you intended, but is commonly done.

To say you want to touch the rifling (or a distance off the rifling) with a 175 eldx at a col of xxx” avoids that misunderstanding.

I‘d ask them about it before you shoot it, and it might be a simple swap out for the barrel you want. I’d emphasize that there wasn’t a meeting of the minds and you did not want this much freebore.

Option 2 is just shoot it as is and it will probably shoot great. A 7 mag doesn’t have a very long accuracy life, 1000 ish rounds, so you might not notice an effect of the long throat if it’s otherwise an accurate barrel.

 

[BrBa]

Any of those 175s you list is definitely a long jump. Only way to eliminate the variable of excessive jump is to seat some long, load them individually and test for groupings.

 

[Buelrdr29]

So just did some testing and got different results than with the hornady case length modified case tool.

Made two dummy rounds. One was a resized case with a lubed bullet and the other a fired case and a loctited bullet. Kept the bullets seated too long and pushed them in with the bolt.

The lubed bullet is 3.395 and had about .060 of rifling marks. So probably touching at 3.455.

The loctite one was 3.367 with about the same marks. So touching at 3.427.

The action opening is only 3.344 and the rounds would not eject. Had to pull the bolt out to extract them.

So finding the jam length minus .020 gives about 3.43 ish. That gives a .140 jump to fit the mag.

 

[Buelrdr29]

I may be wrong, but what you said can be interpreted two ways. One way is just for twist rate, and the second is with custom chambers they might have set the freebore to match the bullet when it’s full diameter base is even with the start of the neck, which is not what you intended, but is commonly done.

To say you want to touch the rifling (or a distance off the rifling) with a 175 eldx at a col of xxx” avoids that misunderstanding.

I‘d ask them about it before you shoot it, and it might be a simple swap out for the barrel you want. I’d emphasize that there wasn’t a meeting of the minds and you did not want this much freebore.

Option 2 is just shoot it as is and it will probably shoot great. A 7 mag doesn’t have a very long accuracy life, 1000 ish rounds, so you might not notice an effect of the long throat if it’s otherwise an accurate barrel.

They did not have a custom reamer for the bullet selection. They just told me today they only use saami spec reamers. I have had the barrel for a couple years now and have been shooting it. Maybe a few hundred rounds or less so far. I did a break in as they suggest and have spent a lot of time trying to find a load that works. I figufed I would try it with the specs they sent assuming they know more than me. Well I feel like I have wasted a lot of time trying to figure it out. One group will be decent and then go out 100 yards and it is sh**. Not sure how such a long freebore happens with a saami spec reamer.

 

[Wrench]

Three capital letters is how.

Chamber cast it and that'll be the answer. To be fair, when I get a reamer from Manson, Clymer or jgs....I trust the print and don't typically cast them.

There's some advice hidden in that statement.

 

[BrBa]

Do those capital letters include a G, a P, and a T, in no particular order?

 

[SDHNTR]

Why not just ask the builder for a chamber print?

 

[wind gypsy]

If you were told you were getting a certain chamber spec and got a different one I can see being upset with the 'smith but that doesn't sound like the case. Beyond that, if it's a bad chamber job or a bad barrel, that could be another thing. Just having a long jump doesn't necessarily mean problems like others stated. weatherby cartridges are designed with jump like that.

Why not just ask the builder for a chamber print?

So they can send a SAAMI print?

 

[wapitibob]

A few hundred rounds on a 7 mag and hot powder is more than enough to burn the throat back.
My 7STW has a factory mag box length of 3.6. My current load is 3.935 and .002 off the lands for a saami chamber.

 

[Pickettpuncher]

They said they only use a saami spec reamer and they had dummy rounds thr reamer was built too. Said it was 3.310 COAL. Mag fits 3.29 which is saami COAL.

Is the sammi number you keep using the cartridge spec or the chamber spec, they won’t be the same