COAL using CBTO from Hornady OAL gauge significantly shorter than book COAL?

[reformedcitykid]

Just use your expander on your "depriming die"

Good idea, I'll do that. Thanks!

 

[FEENIX]

Good idea, I'll do that. Thanks!

Does the bullet drop easily into the modified case as-is? If not, then you need an expander mandrel/die that will allow you to do so.

 

[CajunCowboy]

Your current CBTO measure is less than the saami freebore measure for the 6.5cm (2.140") so very good chance the bullet is getting hung up at the chamber mouth or in the freebore region BEFORE the lands (throat region). Two ways I've run into the same issue are (1) dirty/carbon fouling…rifle needed a good cleaning of freebore and throat area..lots of c4 and brushing (2) new rifle that needed to be polished or in my case broken-in (~200 rds) then cleaned thoroughly. Bore scope can help diagnose the first. I had inconsistent measurements in that instance. With the second type, hammers (but not Bergers) were getting a consistent short reading on a new barrel. The first 200 rds broke the barrel in..likely deburring or polishing a spot where they were hanging up. Hope this helps

 

[skipglo]

I found the easiest and quickest way to find the rifling and then measure the CBTO is to cut 2 1mm slits in the neck, size the case, carefully put it in the chamber, close the bolt lifting and lowering the handle a few times with the ejector removed, then carefully remove it. Measure both OAL & CBTO after this. I normally do this a few times to average results, but rarely does this method alter much.
CBTO is different to COAL, maybe this is your confusion?

Cheers.

X2^^^^^^^ been doing it for years...K.I.S.S.! But with the ejector in.

 

[Philward]

Hi all,
I just got a Hornady OAL gauge and am trying to find the CBTO for my Tikka T3x CTR 6.5cm rifle using a Hornady 6.5cm Modified Case & SMK 140gr bullets. As per Sierra's book, the COAL is 2.810". When I measure the CBTO using the modified case I get 2.0080", which gets me a COAL of 2.5125. This seems significantly shorter than the book COAL.

Has anyone else encountered this? Any thoughts would be much appreciated.

Thanks!

Are you maybe forgetting to re-zero the calipers after removing the comparator?

 

[JAYgs8163]

Amazing how people don't read!

 

[Texis]

I use a caliber diameter mandrel.. ie .284, .308 etc. run it through the brass, the brass will pop back .001-2.
Start a bullet in it, put it in the rifle and use the bolt to seat it into jam. Extract it and measure accurately with an ojive bushings. I typically do 3 for each bullet type. Rarely are they off by more than a couple of thousands. This method has been excellent for me for years… very accurate "to jam" measurement.
Lube the ojive where it will contact the lans.
Goods luck

 

[8x68s]

Interesting. All my modified Hornady cases are quite liberal…like no neck tension al all. Very loosely goosey.

I had a Hornady 260 Rem measurement case that wouldn't accept a bullet at all. They must have forgotten to run the oversize mandrel thru the case at production. I tried to fix it with a .270 mandrel but it was too much.

 

[lancetkenyon]

Fellas, thanks for the input! To clarify, I measured CBTO using the Hornady OAL gauge, then removed the comparator from the calipers and measured COAL while still attached to the OAL gauge, so I'm not confusing the 2 numbers. Yes, I am trying to find the lands. And yes, I understand that book COAL is an estimation and every rifle is different, which is why I'm trying to find my rifles' measurements to go off of. However it is a good basic reference number and being .3 off raised some flags.

However, after fiddling with it a bit more and using a few different types of bullets, I think the issue lies with the modified case. It seems there is too much neck tension causing the bullet to meet what feels to be the lands. The bullets are not able to fall freely from the OAL if I turn it upside down. I'm going to sand/polish the inside of the neck to see if that works and will report back. Thanks!

Did you reset zero on your calipers with and without comparator installed?

 

[slikmrc]

Give the modified case mouth a little inside neck chamfer and see if that helps. Also you could load up a dummy round to Sierra's OACL and try it in the gun.

 

[cajun]

Something is off. The bullet should slide into the modified case easily.

 

[JAYgs8163]

He's already mentioned gents that the neck of the case gauge is too small and binding. He needs to open it up some. The end

 

[reformedcitykid]

Gents, glad to report back with some good news! As suspected, it was an issue with the modified case neck. I sanded it down until the bullet slid freely, and took 10 measurements:
2.3230"
2.3270"
2.3260"
2.3275"
2.3270"
2.3260"
2.3250"
2.3230"
2.3245"
2.3260"
For an average of 2.3256" aka 2.3255"

Any recommendations how far off the lands to start loading? I've read from .01" to .04", if anyone has a compelling argument one way or another please let me know!

Once again, thanks for everyone's input!

 

[FireFlyFishing]

Gents, glad to report back with some good news! As suspected, it was an issue with the modified case neck. I sanded it down until the bullet slid freely, and took 10 measurements:
2.3230"
2.3270"
2.3260"
2.3275"
2.3270"
2.3260"
2.3250"
2.3230"
2.3245"
2.3260"
For an average of 2.3256" aka 2.3255"

Any recommendations how far off the lands to start loading? I've read from .01" to .04", if anyone has a compelling argument one way or another please let me know!

Once again, thanks for everyone's input!

I've found from my testing and not forums…0.020" to start.