Throat Length Question

[CouesPursuit]

Please forgive my lack of knowledge here.

 

In my 7mm with 168 gr LRABs, my seating depth test produced a throat OAL of 3.42" to the lands, which is .13" > SAAMI.

 

I bought a new .223 and performed the same depth test with a 65 gr SGK, which reveals a 2.25" throat length, which is .01 < SAAMI.

 

Am I wrong in assuming the chamber throat should be equal to or exceed SAAMI specs?

 

At this point I would probably load 2.25" OAL and shoot. Thoughts on the 0.01 variation?

 

Thank you.

[STOMP442]

Bullet weight and design has great effect on bullet length. The ABLR bullets are a heavy for caliber bullet with a secant ogive design, meaning the nose of the bullet is much longer and sleeker than a normal tangent ogive design such as the game king. Do the same measurement process with a sierra game king in your 7mm and you will end up much closer to the standard measurements.

[CouesPursuit]

Got it! CBTO most likely covers this more effectively?

[ctracingraptor]

I find that interesting, i load 69 SMK to 2.260, loading a heavier bullet will definatley change your oal, with a short throat like that running something like a 77 SMK wont even be a option. What method are you using to measure your oal?

[CouesPursuit]

I'm chambering an unsized cartridge marked with a sharpie on the SGK. Didn't need the sharpie as the bolt smoothly closed and ejected the cartridge with seated bullet five times and came up with-

2.26

2.25

2.25

2.26

2.25

[ctracingraptor]

I dont have any sgk lying around to compare to, but looking on the ol interwebs, the sgk appear to have a larger ogive, and that might be your issue. Im curious if you wouldnt have this issue with a 69 smk, since that are much narrower.

[lancetkenyon]

As long as they feed smoothly, fit in the magazine, and shoot well, CBTO and COAL are not the end all goal.

[WampusCat]

Far more important than the final length you end up with is that you develop a system to accurately and repeatedly measure and record the data you build the cartridges with. Let the gun/bullet tell you what it wants as far jump to the lands. The book and SAAMI are merely a baseline to fit the masses.

 

On occasion, some things like heavy for cal bullets confined to the internal dimensions of a magazine will limit or mandate your OAL or in your case it might be a short throat. In most cases I let an OAL gauge and the bullet I intend to use determine a starting point of touching or .010" off the lands regardless of where the book recommends, then compare to mag length constraints for final starting point.

 

Also note that you will build pressure much faster as you increase charge weight if you are jamming a bullet into the lands.

 

All this is assuming you are reloading to find and tune the most accurate recipe possible and not just making bullets that shoot.

[CouesPursuit]

Got it and thank you all!

[PowellSixO]

Far more important than the final length you end up with is that you develop a system to accurately and repeatedly measure and record the data you build the cartridges with. Let the gun/bullet tell you what it wants as far jump to the lands. The book and SAAMI are merely a baseline to fit the masses.

 

On occasion, some things like heavy for cal bullets confined to the internal dimensions of a magazine will limit or mandate your OAL or in your case it might be a short throat. In most cases I let an OAL gauge and the bullet I intend to use determine a starting point of touching or .010" off the lands regardless of where the book recommends, then compare to mag length constraints for final starting point.

 

Also note that you will build pressure much faster as you increase charge weight if you are jamming a bullet into the lands.

 

All this is assuming you are reloading to find and tune the most accurate recipe possible and not just making bullets that shoot.

 

Good information here.

 

It's worth spending the money on a hornady bullet comparator set, and a hornady lock n load oal gauge (straight for a bolt gun or the auto version for an AR). This way you can measure the Cartridge base to ogive length, and determine the exact seating distance for a particular bullet to the lands. Especially if you plan on loading rounds near the lands. If not you can very easily load one too long, and jam it into the lands. If you're working with hot loads, this can equal a very dangerous situation. I know, because I've done it. Even with these tools I made the mistake of not measuring every round as I seated my bullets. On one particular lot of bullets I got for my 7mm, I measured the first few rounds I loaded, and then proceeded to load 50 rounds. Unfortunately a few of the bullets in the box I got measured .010-.015" longer at the ogive than the rest. I load my rounds .001" off the lands. Needless to say, I jammed one before I learned my lesson. It blew the primer out of the case, and collapsed my ejector spring. Luckily this was the only harm done. But it could have been very bad. Since this experience I measure every single round after I seat the bullet. It's more time consuming, but much safer.

 

 

 

 

 

CBTO length seated to lands.

 

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Round loaded too long, due to bullet being slightly longer at the ogive.

 

]

[naturebob]

Throat length! wasn't same topic I was thinking of!................BOB!

[Titanium700]

So?

Not this?

[lancetkenyon]

SAAMI specs are for cartridge length, not necessarily chamber throat depth. The chamber for case dimensions match, but not necessarily the throat. Which is why all guns shoot differently. Manufacturers can have vastly different reamers, some new, some worn, some cut just a touch different, some tight, some sloppy. Also why custom rifles usually have a lot tighter chambers, and the leade, or throat, can vary even more per the customer request for bullet choice and magazine length. If you want a rifle throated for the heavier bullets, they will usually have a longer jump to lands when using a factory cartridge. Factory cartridges are made to work in every gun chamber and magazine with that chambering 99% of the time. That is why you usually see hand loaders with COAL and CBTO longer than SAAMI. They are chasing the lands in THEIR gun for accuracy, powder charge/pressure, and consistency. Not loading to fit in ANY gun.