Chamber dimension question

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hawk-i

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Looking for chamber dimension info.

Cartridge 300 PRC

Looking up the specs for the cartridge, Sammi says base/head diameter of .532"

Looking up the specs for the chamber, Sammi says base/head area diameter of .533"

The question is, should the chamber always measure at least a min. of .533 even with a so called match chamber or is there any situation where this measurement can be less?

Reason for question is I'm getting sticky bolt after second firing on new brass (no pressure signs on base of brass).

Light bolt sticking with brass measuring .5311".... .5306" feeds nice.... .5322 bolt 1/8" from locking the lugs into place (.5322 this is all my Whidden dies will resize to, Hornady dies will only resize to .5326).

But still even at the Hornady dies sizing of .5326 the cases should still chamber nicely, shouldn't they?
 
Your question - to be SAAMI compliant, the chamber should be sized as per their specs. Nothing says that you or another maker can not do something differently - is just not SAAMI compliant at that point. Might want to review that drawing - I am pretty certain most SAAMI dimensions come with a plus/minus tolerance - but that might be a dimension where they give no tolerance to it.

Go here: https://saami.org/wp-content/uploads/2018/07/300-PRC-Public-Introduction.pdf There appears to be tolerances given for both cartridge and in the call out for the chamber.
 
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saami plus minus, you could have a minus chamber and plus dies resulting in tight brass, but first you have to find out where it is tight, draw lines down a case with a sharpie and chamber it, see where the pen mark is rubbed, this will answer more questions in regards to base to large, shoulder not pushed back far enough, which shell holder are you using when resizing your cases? RCBS #4 or #38, the proper shell holder being #38
 
yodave said:
saami plus minus, you could have a minus chamber and plus dies resulting in tight brass, but first you have to find out where it is tight, draw lines down a case with a sharpie and chamber it, see where the pen mark is rubbed, this will answer more questions in regards to base to large, shoulder not pushed back far enough, which shell holder are you using when resizing your cases? RCBS #4 or #38, the proper shell holder being #38
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Shoulder is pushed back .002... Redding #5 I think ....rub marks in the head/base area...if I'm reading it right Sammi drawings says +.002 for body dia. so shouldn't be less than .533 but can be up to .535(not sure on this)????
 
Potashminer said:
Your question - to be SAAMI compliant, the chamber should be sized as per their specs. Nothing says that you or another maker can not do something differently - is just not SAAMI compliant at that point. Might want to review that drawing - I am pretty certain most SAAMI dimensions come with a plus/minus tolerance - but that might be a dimension where they give no tolerance to it.

Go here: https://saami.org/wp-content/uploads/2018/07/300-PRC-Public-Introduction.pdf There appears to be tolerances given for both cartridge and in the call out for the chamber.
Click to expand...

Yes, thats the drawing I was looking at.
 
hawk-i said:
Shoulder is pushed back .002... Redding #5 I think ....rub marks in the head/base area...if I'm reading it right Sammi drawings says +.002 for body dia. so shouldn't be less than .533 but can be up to .535(not sure on this)????
Click to expand...

yes, can be up to 0.535

do you have click dies? is so try bumping the shoulder back 2.5 thou and retest, but before doing that, run a case into the sizer die, drop the ram down and turn the case half a turn, and size it again and then try the brass in the chamber, some brands of brass are terrible for spring back and sizing twice sometimes cures it
 
yodave said:
yes, can be up to 0.535

do you have click dies? is so try bumping the shoulder back 2.5 thou and retest, but before doing that, run a case into the sizer die, drop the ram down and turn the case half a turn, and size it again and then try the brass in the chamber, some brands of brass are terrible for spring back and sizing twice sometimes cures it
Click to expand...

I've bump the should back as much as 4.5 thousand, makes no difference...when sizing I have been turning the case 1/3 rev and sizing 3times for this pos rifle.

Don't have click die but using Redding competition shell holders.

I think my chamber is less than .531 in the head area from measuring the cases that stick.
 
Not real certain that if you can get something out of a cylinder, that it is the ID of that cylinder - else it would be too tight to remove? I think cerrosafe casting is traditional way to measure a chamber diameter - it shrinks when cooled, so can be removed, then expands to original diameter after 60 minutes - so get the correct measurement that many minutes later. Not saying that your conclusion is not correct - but your process used to get there is not the "normal" way to measure an ID.
 
hawk-i said:
I've bump the should back as much as 4.5 thousand, makes no difference...when sizing I have been turning the case 1/3 rev and sizing 3times for this pos rifle.

Don't have click die but using Redding competition shell holders.

I think my chamber is less than .531 in the head area from measuring the cases that stick.
Click to expand...

Measure the bases of your brass, that will tell you pretty fast if your chamber is on the small side.

I'm willing to bet that rifle is one of the most accurate in your inventory?

I have a rifle, 6.5x57, that I have to full length resize the case after every shot. The chamber in this rifle is right on minimum.

It's very accurate.

What are you describing as ''sticky'' bolt lift?

The 6.5 PRC is a high pressure cartridge and brass is going to flow. You know that already.

As mentioned, you may have to resize the side walls and base a bit more, maybe as much as .005, which is what I have to set back the shoulders on my 6.5x57.

If push comes to shove, you may need a small base sizing die.

If I had the option of full length resizing every time to get excellent accuracy, it wouldn't bother me one bit.

If you don't see any signs of pressure, such as a ring INSIDE the case, you will likely be able to get several reloads on that brass, with some annealing every 3-5 loads.
 
redding comp shell holder set gets taller so you size less, they don't go shorter then a standard shell holder to take you under SAAMI spec, for that you need to file a shell holder. if your chamber is that tight you may have to go to custom sizing dies to match the chamber, the odd time you can get lucky and buy another sizing die that is smaller at the base then the one you're using, but unless you can open every die box and check your brass in the dies you looking for a needle in a haystack, and I am unsure if anyone is making a small base sizing die for the 300 as of yet, or if they ever will, so for now, it's either custom die, or a polishing on the chamber to make it closer to what your dies are capable of sizing to
 
bearhunter said:
Measure the bases of your brass, that will tell you pretty fast if your chamber is on the small side.

I'm willing to bet that rifle is one of the most accurate in your inventory?

I have a rifle, 6.5x57, that I have to full length resize the case after every shot. The chamber in this rifle is right on minimum.

It's very accurate.

What are you describing as ''sticky'' bolt lift?

The 6.5 PRC is a high pressure cartridge and brass is going to flow. You know that already.

As mentioned, you may have to resize the side walls and base a bit more, maybe as much as .005, which is what I have to set back the shoulders on my 6.5x57.

If push comes to shove, you may need a small base sizing die.

If I had the option of full length resizing every time to get excellent accuracy, it wouldn't bother me one bit.

If you don't see any signs of pressure, such as a ring INSIDE the case, you will likely be able to get several reloads on that brass, with some annealing every 3-5 loads.
Click to expand...

When I mic. the base, any brass over .531 will stick...by stick I can open the bolt all the way but need to give a good slap the bolt handle with my palm to eject the brass.

Yes accuracy is good ... I do anneal and full length resize every time
 
hawk-i said:
When I mic. the base, any brass over .531 will stick...by stick I can open the bolt all the way but need to give a good slap the bolt handle with my palm to eject the brass.

Yes accuracy is good ... I do anneal and full length resize every time
Click to expand...

I find your description interesting - an action with NO primary extraction, at all. I did not know there was such a thing - I am old, I guess!!! Most all of the mil-surps that I play with have some rearward motion of the fired brass, from the leverage of bolt handle when opening the bolt. What you have must be a modern accuracy thing? I notice from the SAAMI chamber drawing that the chamber should have a very slight taper - so any rearward movement of the fired case should result in free extraction, I would have thought?

A really wild thought - is there any possibility that you have some scum or grunge in your chamber - with a light to help look, the chamber walls should look bright and shiny like a mirror - no scum, no marks, at all. Dry as can be - no lube, nothing - just very shiny smooth in there.
 
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RJ posed a good question - OP reported several dimensions in Post #1 to four decimal places - I find that very difficult to get - using a Mitutoyo micrometer I find it very difficult to measure the same thing three times and to get even two exact same readings, to the fourth decimal place. I am not good or experienced at it - why I should be more "suspect" of dimensions to that level of precision - I can not replicate those ...

I am reasonably confident "to the thousandth" (three decimal places) with the Mitutoyo. But my NAPA digital callipers said on their box they are accurate "to .001" - despite that their readout displays 4 decimal places - the unit is simply not capable of that level of accuracy - the maker said so ...
 
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remington jim said:
hawk-i : When I mic. the base, any brass over .531 will stick...by stick I can open the bolt all the way but need to give a good slap the bolt handle with my palm to eject the brass.

My NEW unfired Horn brass is .531 - How can your fired be only .531 WTH ? RJ
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I get <.530 on my new ADG brass with my micrometer and right on .530 with Lapua brass. The same micrometer (mastercraft) gets .308 on both berger and hornady 30 calabre bullets...so I think its measuring pretty accurately.

However, I ordered a new Mitutoyo micrometer this morning that is supposed to be accurate to .00005" :)
 
Remington jim,... to add to that I just measured new unfired 6.5 PRC hornady loaded ammo and it measures .530 bang on
 
hawk-i said:
I get <.530 on my new ADG brass with my micrometer and right on .530 with Lapua brass. The same micrometer (mastercraft) gets .308 on both berger and hornady 30 calabre bullets...so I think its measuring pretty accurately.

However, I ordered a new Mitutoyo micrometer this morning that is supposed to be accurate to .00005" :)
Click to expand...

OP - your Mitutoyo will come with (or used to come with) a "standard" that is used to calibrate the instrument - I hope they still come with that - is a ground surface thing - very precise - they used to specify a very specific temperature for "accuracy" - at the level of precision of 4 or more decimal places, you need to be using some sort of callibration device. I doubt those bullets at .308" will be consistent enough for four places accuracy.
 
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Potashminer said:
I find your description interesting - an action with NO primary extraction, at all. I did not know there was such a thing - I am old, I guess!!! Most all of the mil-surps that I play with have some rearward motion of the fired brass, from the leverage of bolt handle when opening the bolt. What you have must be a modern accuracy thing? I notice from the SAAMI chamber drawing that the chamber should have a very slight taper - so any rearward movement of the fired case should result in free extraction, I would have thought?

A really wild thought - is there any possibility that you have some scum or grunge in your chamber - with a light to help look, the chamber walls should look bright and shiny like a mirror - no scum, no marks, at all. Dry as can be - no lube, nothing - just very shiny smooth in there.
Click to expand...

No, there is primary extraction but it doesn't move the bolt far enough back to get past the sticky part on the head area of the case...the marks on the case a pretty easy to see, no need to use marker pen.

I'll see if I can see anything in the chamber, thanks
 
I am with Potashminer on this one, if you can open the bolt all the way open then have to slap it to get the brass out, you have no primary extraction, and this issue on a factory rifle is very true to two leading manufactures, remington, and savage, the savage is easy to time with a file, the remington requires a gunsmith and the handle relocated..........
 
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