Odd distance to the lands..

[Grimm]

Okay so I got a 300 PRC Christensen Mesa a while back. I'm just getting around to working a load up for it before elk hunting and while trying to come up with a seating depth, I made an odd discovery. The lands in my rifle are at 2.910 based on my hornady gauge. I measured this literally 10 times thinking I was doing something wrong and the result was always the same. This seemed short so I measured a factory Precision Hunter 212gr eldx and came up with a measurement of 2.915. This suggests that I'm jammed into the lands on factory ammo... Additionally, if I load .010 off the lands with my 200gr Terminal Ascent bullets, I'm under the min OACL specified by SAAMI. I'm not really sure what to make of this yet as I haven't fired any test loads yet but I wanted to know if anyone else finds this strange. I am trying to look through the drawings on saami's website but I'm having a hard time figuring out what numbers I should be looking at.

The factory ammo didn't seem to be showing pressure signs in the form of flattened primers and as far as bolt lift goes, it's a brand new rifle so I have no baseline. I started my load workup halfway between min and max and I'll definitely be making some loads all the way down to minimum now to be on the safe side.

Can anyone tell me based on these drawings what distance I'm supposed to have to the lands? This just seems too short and really hamstrings my ability to load for it. saami

I found a post on another forum where people seemed to be having the same types of issues with their guns having super short chambers also so I'm not super optimistic..

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[Ura-Ki]

Hummm,...........................Not experienced with this particular chambering, but.......
According to the chart you supplied,, i'm seeing a case length of 2.580, and a total loaded length of 3.575 min to 3.700 MAX, and a chamber depth to free bore ( nominal ) of 2.8368 out to 3.005 So, i'm not sure where your running fowled, are you measuring total length here or are you extrapolating base case length with bullet seated depth to arrive at, What? Assuming your measuring correctly, and have arrived at 2.910, you shouldn't be fully into the rifling until 3.005 at the minimum and should still have almost 6 thousandths, (which to me is about right, if slightly tight) so, have you loaded a max length dummy with your chosen bullet and actually measured exactly where your chamber specs out and where the lands start? I would start with that info, find where in the acceptable range you land and adjust from there!

 

[Grimm]

Hummm,...........................Not experienced with this particular chambering, but.......
According to the chart you supplied,, i'm seeing a case length of 2.580, and a total loaded length of 3.575 min to 3.700 MAX, and a chamber depth to free bore ( nominal ) of 2.8368 out to 3.005 So, i'm not sure where your running fowled, are you measuring total length here or are you extrapolating base case length with bullet seated depth to arrive at, What? Assuming your measuring correctly, and have arrived at 2.910, you shouldn't be fully into the rifling until 3.005 at the minimum and should still have almost 6 thousandths, (which to me is about right, if slightly tight) so, have you loaded a max length dummy with your chosen bullet and actually measured exactly where your chamber specs out and where the lands start? I would start with that info, find where in the acceptable range you land and adjust from there!

I'm measuring 2.910 base to ogive as being the point where the bullet is enough in contact with the lands for it to be stuck and needing me to pop it back out with a dowel. I'm using a Hornady oal gauge with their modified 300 prc case to come up with this number. If I'm loading .010 off lands with this bullet, I'm .007 under minimum length per saami.

Im not sure if that clears up any confusion for you or not. I guess the chamber depth to freebore measurements are the ones I'm confused about

 

[Ura-Ki]

Sounds like the bullets them selves might be the issue if the olgiv is hitting the lands seated that far back! Have you tried any other bullets to see where they hit?

 

[gmerkt]

Can anyone tell me based on these drawings what distance I'm supposed to have to the lands?

Far enough that they aren't touching and you already know this.

I've owned a lot of rifles and loaded my own ammo for most of them. I've had a few that were just plain short throated. I can recall one that was a Remington 760 in .35 Rem. and another was a Browning Model 53 in .32-20. The latter was a known known; the former must've been a one-off because there wasn't anything rare or unusual about the rifle or caliber.

So when you have one of these, the SAAMI standards can go out the window. You're talking thousands of inches, it's very unlikely that you'd encounter an over-pressure situation by seating the bullet slightly deeper. Meaning, by diminishing your case volume. You'd be way more likely to cause over-pressure by jamming a bullet into the leade of the rifling.

In the two examples I mentioned having, one was a pump rifle, the other was a lever gun. So I wanted to use the cannelure for crimping the case. So seating the bullet deeper wasn't a remedy. My solution was to trim the cases back several thousandths to move the bullet away from the leade. Uh-oh, there goes SAAMI out the window again.

Yes, bullet ogive design can be a contributing cause. Hornady bullets often use a tangent ogive. Which has a short section that goes straight out before the curve starts. If you have a short throat, the straight section will jam into the leade. The secant ogive starts the curve typically right at the case mouth or fairly close, which gives a bit more space off the leade.

Another solution is to have a gunsmith cut the leade a little deeper.

I don't even know what a 300 PRC Christensen Mesa is, but theory is theory regardless of cartridge.

 

[Grimm]

Sounds like the bullets them selves might be the issue if the olgiv is hitting the lands seated that far back! Have you tried any other bullets to see where they hit?

Just tried some 180 gr accubonds and they're quite a bit further forward. That confuses me though because shouldn't the base to lands/ogive measurement always be the same regardless of bullet? I understand oal will change but you'd think base to lands would be constant

 

[Ura-Ki]

Just tried some 180 gr accubonds and they're quite a bit further forward. That confuses me though because shouldn't the base to lands/ogive measurement always be the same regardless of bullet? I understand oal will change but you'd think base to lands would be constant

Not always, see gmerkts reply above! Olgive can vary radically between manufactures, and it's not uncommon to find it very far forward in the quest to get high B.C.'s assuming most will be handloading and the rifles manufactures giving tight/short chambers to allow a good smith to ream to the desired O.A.L. for a given type/design bullet! Now were getting serious!

 

[DizzyJ]

Your bullet shape will have EVERYTHING to do with distance to the lands.

Longer sleeker Bullets will be able to be seated out further than shorter, fatter (stubby) profile bullets will. The shape of the ogive will absolutely change your numbers.

 

[DizzyJ]

You can take that factory ammo and smoke the bullet (candle)(or black sharpie) and chamber it to see if it actually makes contact with the lands if you're worried about that.

 

[gmerkt]

The Swiss K31 is another example that comes to mind re. short throats. Those ALL have a short throat by design. The Swiss GP11 ammo had a very sharply sloping spitzer bullet. Reloaders need to be careful selecting bullets for those.

 

[Grimm]

Thanks for all the replies, guys. This is the second bolt gun I've reloaded for and the first had the throat the length of a football field so this is new to me. I'm going to have to do some more reading about how the ogive shape changes distance to lands.. I guess my understanding of the ogive was wrong.

I always figured the ogive was where the bullet became full diameter and that was the point it contacted the rifling. Sounds like the bullets contact before full diameter which is why the shape of the ogive has an effect on distance and why that distance is not a constant. Am I following?

 

[DizzyJ]

Thanks for all the replies, guys. This is the second bolt gun I've reloaded for and the first had the throat the length of a football field so this is new to me. I'm going to have to do some more reading about how the ogive shape changes distance to lands.. I guess my understanding of the ogive was wrong.

I always figured the ogive was where the bullet became full diameter and that was the point it contacted the rifling. Sounds like the bullets contact before full diameter which is why the shape of the ogive has an effect on distance and why that distance is not a constant. Am I following?

The comparator may or may not hit EXACTLY on the ogive. Also there is an angle, or leade machined into the opening of the comparator that may or may not match up with the angle of the ogive on the bullet. This will give you different readings for different bullet profiles. Even different lot numbers for the same bullet and manufacturer can give different readings.

 

[Grimm]

The comparator may or may not hit EXACTLY on the ogive. Also there is an angle, or leade machined into the opening of the comparator that may or may not match up with the angle of the ogive on the bullet. This will give you different readings for different bullet profiles. Even different lot numbers for the same bullet and manufacturer can give different readings.

That makes sense. I noticed too that these terminal ascents have a much different profile that the eldx and accubonds. They're almost cone shaped whereas the former are both a more rolling taper.

 

[DizzyJ]

That makes sense. I noticed too that these terminal ascents have a much different profile that the eldx and accubonds. They're almost cone shaped whereas the former are both a more rolling taper.

Correct those are the different ogive shapes/profiles.

 

[Grimm]

Okay, 1 last question. Testing will probably answer this but I'll ask anyway - is there a disadvantage to me using these bullets even though over half of the bullet is seated inside giving me an oacl 070 shorter than the powder manufacturers length when they did their load work ups?

 

[DizzyJ]

Okay, 1 last question. Testing will probably answer this but I'll ask anyway - is there a disadvantage to me using these bullets even though over half of the bullet is seated inside giving me an oacl 070 shorter than the powder manufacturers length when they did their load work ups?

As always, start low and work up. As long as you're not seating the ogive below the end of the case and you drop your powder charge you should be fine.