308 Win...Does this seating depth look right (with pictures)? Optimal trade-off ?

Munkey1973

Munkey1973

CGN frequent flyer
Rating - 100%
79   0   0
Location
Regina, SK
I am building my first 308 F T/R rifle (first ever 308 for that matter). Custom barrel from Krieger should hopefully be here next month and my gunsmith is asking me about reamers.

I got a minimum SAAMI spec reamer picked out that has a very short freebore … might be too short. So if need be, the throat will be reamed further using a throat reamer.

In the meantime, I'm currently trying to create a dummy round based on my desired seating depth.

The projectile that I will be using by the way, is the new Sierra 200 MK (#2231) with some Lapua 308 Palma Brass.

Looking at two methods for creating this dummy round:

(1) Erik Cortina's Method https://www.youtube.com/watch?v=U9XpnpgJd0A&list=PLJm6jq3qGs_r_0FXPLnC2gnaSI5STs1oc&t=0s&index=3 A technique that is generally applicable to any cartridge/component combination...Lining up the bottom bearing surface with the intersection of the neck and shoulder..... I had to sacrifice some a bullet and brass, but this method makes sense to me...with a cut-a-way I can see what is going on inside of the case. The visibility on the work bench (versus the video) worked better than expected.

(2) Sierra's Load Data https://sierrabulletsblog.com/2018/01/31/sierra-bullets-load-data-for-2018-new-matchking-bullets/…. Very specific to the projectile I intend to use. On their site they recommend a COAL = 3.150. Well hey, it is what the manufacturer recommends after all and one should think they kinda know what is best practice for their product.

The pictures below show the end result of Sierra's method...remember this when looking at pictures.

With Sierra's Load Data, the bottom boat tail is inline with the body/shoulder intersection (as noticeable in the 2nd picture). I should have flexibility on how much powder I wish to use. However, the bullet seems way too far out to me. The bearing surface length is almost half way up the case neck! (as displayed in the arrow gaps in the 1st picture) Might be previously misguided, but I think that it is paving a road to poor run-out and potential doughnuts (regardless that my outer case necks will definitely be turned anyways)?

The Cortina Method guarantees (damn near) that the entire Bearing Surface Length of the bullet with be in contact with 100% the case neck of the brass. Which thereby eliminates my fears of bad run-out and future doughnuts from using Sierra's Load Data (at least in my mind). But these new Sierra 200's have a very long boat tail. Although I did not take a picture of the Cortina Method end result... imagine that the first 2 arrows on the left will overlap each other and the black marks on the bullet were covered up the case..... the bullet will displace a lot of volume from the case. So I am concerned that I will get a weak powder charge that is insufficient for F class shooting.

Option 1 Cortina Method PROS: save my brass, achieve low run out... CONS: run the risk of a competing with sub-sonic projectile that an electronic target can't register at the 1,000 yard shooting line ?

Option 2 Sierra's Load Data PROS: flexibility with powder charge CONS: get banana cartridges & have to invest a inside neck reamer next season ?

Option 3 ... split the difference... Move the middle arrow half the way to the left arrow ?


depth_zps8yzdb64t.jpg


side%20by%20side_zpswdeum55e.jpg
 
Sorry. Didn't realize your trying to create a dummy round. I would place the top of the boat tail at the shoulder junction so you have a decent hold of the bullet, and will allow you to chase the lands as you shoot high volume. I guess I would use the Cortina method
 
nevanevan said:
Sorry. Didn't realize your trying to create a dummy round. I would place the top of the boat tail at the shoulder junction so you have a decent hold of the bullet, and will allow you to chase the lands as you shoot high volume. I guess I would use the Cortina method
Click to expand...

LOL, yeah I was about to reply to your previous post. Its for a dummy round, the barrel is still on its way. Gunsmith tells me that he could have a reamer made custom for me. But sounds expensive, so I will just go with shorter freebore and extend it as needed.

Like I said, I didn't take picture of Cortina method (pic. may not show how deep the boat tail goes into the case). I think when I have time today, I will tape off the Dremel cut as to make a see through window. Make say a 42 grain charge, dump it in. Then re-seat the big 200 bullet and see how much space is in the load cartridge.
 
You don't seat bullets to your desired seating depth. You seat the bullets to the length that your barrel desires. COAL is a poor way to measure how much jump into the lands the bullet will have in your chamber.
 
Precisely as nevanevan said its a dummy round.
I want to build a cartridge to what I think will produce the best performance.
Then the chamber will be designed to match the dummy cartridge.

In a store bought rifle, I would be designing my ammo to match the existing chamber..... In this case, I will be creating a custom chamber (throat really) to match my ammo.

The question boils down to where do I want the middle arrow to go?
 
I have found the more you engage the neck, the more accurate. old rule - engage the diameter width into the neck bearing depth. as stated, as the throat wears, you can chase it.
 
Start with the boat tail transition at or near the neck/shoulder junction. Throat for a .015 jump. Don't overthink it. As the throat wears, you'll be able to chase the lands and still have lots of bearing surface in the neck to work with. I've heard some FT/R shooters report up to .035" erosion in the first 1000 rounds so check your touch point every once in a while. Some guys will throat longer to get more powder space but they're not giving themselves any room to compensate for throat wear and they end up retiring the barrel early. That's the trade-off.
 
Last edited:
1-trident-1 said:
I have found the more you engage the neck, the more accurate. old rule - engage the diameter width into the neck bearing depth. as stated, as the throat wears, you can chase it.
Click to expand...

I haven't heard of the rule. So you are saying that about 0.308" of the Bearing Surface Length should be seated into the neck?
 
Munkey1973 said:
I haven't heard of the rule. So you are saying that about 0.308" of the Bearing Surface Length should be seated into the neck?
Click to expand...

Yes. Like he said, just a rule of thumb, gives enough grip to the bullet to prevent being knocked askew by normal handling or when being chambered.
 
Viper26 said:
Start with the boat tail transition at or near the neck/shoulder junction. Throat for a .015 jump. Don't overthink it. As the throat wears, you'll be able to chase the lands and still have lots of bearing surface in the neck to work with. I've heard some FT/R shooters report up to .035" erosion in the first 1000 rounds so check your touch point every once in a while. Some guys will throat longer to get more powder space but they're not giving themselves any room to compensate for throat wear and they end up retiring the barrel early. That's the trade-off.
Click to expand...

That is an interesting perspective. Thanks, I know what you are saying and definitely something to factor in.

In my experience with my (now sold) 223 Rem F T/R gun was that I had 0.006 wear on the throat after the first 1,000 rounds, and then about another 0.004 wear on the next 750 rounds. The gun when to a new home 3 weeks ago and I had it for just over 2,000 rounds through it. I didn't check throat wear on the last 250 rounds from last season.

Man, I wonder how hot those other guys were running to get 0.035 wear after 1,000 rounds ! LOL
 
Viper26 said:
Start with the boat tail transition at or near the neck/shoulder junction. Throat for a .015 jump. Don't overthink it. As the throat wears, you'll be able to chase the lands and still have lots of bearing surface in the neck to work with. I've heard some FT/R shooters report up to .035" erosion in the first 1000 rounds so check your touch point every once in a while. Some guys will throat longer to get more powder space but they're not giving themselves any room to compensate for throat wear and they end up retiring the barrel early. That's the trade-off.
Click to expand...

I'm with you on this one. This is what I prefer for optimal neck bearing. My 6.5x47 lapua has had 0.040" of throat erosion in the first 1200 rounds. And I do not heat the barrel up on it. After this year I'll have to see about getting the chamber set back or I'll be having difficulty reaching the lands.
 
You must log in or register to reply here.
Back
Top Bottom