Help me understand freebore. 7 SAUM 180 ELDM

[JakeC]

Hi all, thanks for your time. I can't seem to find a clear answer to what's puzzling me. The short version is this: Is freebore/lede the same exact thing as the distance at which the bullet contacts the lands?

In other words is the end of the freebore the point at which the taper starts and the ogive sits past the end of the freebore? If so, how far? maybe the angle is such that there's little difference.

I thought I could glean answers from the reamer drawings but I'm smart enough to admit when I have no idea what I'm looking at.

The long version is that I'm trying to determine whether or not to spend the extra money and time to have a prefit barrel throated to a dummy. It's not a huge expense, I just would rather spend the money on something else if the SAAMI reamer will get me out close to 2.95 or more. What I'm building is a 7 SAUM. I thought I had everything figured out but now that I have my components I can see that if I seat the ogive at the end of saami freebore, 90 thou, then the ogive is barely out of the case. This is obvious, I know, I just couldn't grasp it til I saw it. What's worse is that with a 180 ELDM the COAL is only around 2.81. That's shorter than the factory length for a core-lokt, and far short of my mag length. I know I might need to custom throat to get the most out of my mag but I didn't think I'd be so far off. And I'm okay with some bullet in my case body, but this is a little extreme, especially when I move to the 175 ELDX

Anyway, maybe someone can tell me right away if I've twisted myself up or if I just ran into facts and need to change my plan. Thanks!

 

[Jud96]

I asked basically the same exact question on here and another popular forum a little while ago. From my observations and what others pitched in, it's very very hard to nail down the freebore to be exactly what you want. Because like you said, there's a small taper (lead) right after the end of the freebore. So the math is very involved and changes with different ogives on different bullets. From studying reamer prints and my loaded rounds this is what I do. I typically shoot Berger bullets so I use their bullet list to figure out the bearing surface length of the bullet. I subtract the neck length of the cartridge case then add .030-.050 to get above the neck shoulder junction. Then what's left I subtract 0.020 from for the angle just ahead of the case mouth then subtract 0.020 for the angle beyond the freebore. What's left is how much freebore I would want. This is all a guesstimate though. You don't really know until you experiment with ordering and testing multiple reamers. I'm not on that level yet, so for now I do the above method to get an idea.

 

[javman]

Me personally would want to squeeze out the most case capacity out of the Saum. So by going with a dummy round the person or company doing the chambering will set the freebore accordingly. Just my .02 worth but you need to see what bullets your planning on using and checking the different over all length to ogive with each and look for a happy median.

 

[Rhovee]

.235-.245 is what I would use. I had one. But I wouldn't shoot the 180 eldm. Especially if you plan to shoot animals with it. To fragile.

 

[can1010]

first you have to decide what is the max length you can get in your mag or going single shot.
while .090 seems short you really need to keep that for a short action with a 2.9 mag but if using long action or singleshot you can throat it with a .225-.250 freebore. mine has a .228 and while good for bergers and some bullets in the 160-168 class bullets you really need .245-.250 for the elds. all the different bullets will end up 3.0 -3.150 oal. with the .225 and longer freebores.

best bet is a dummy round at max mag length so your distance off lands will leave room in mag.

 

[JakeC]

.235-.245 is what I would use. I had one. But I wouldn't shoot the 180 eldm. Especially if you plan to shoot animals with it. To fragile.

No I don't plan on shooting large animals with them but right now the 180 ELD and 175 ELDX are pretty much all I can find. They're good enough and cheap enough to get me on the right track for ABLR 175 which I ultimately want but cannot find.

Now that measurement you give, is that for an optimum setup regardless of length or is that fitting in a magazine? My savage mag measures a hair over 3" but to me .235 sounds way too long for that. As far as my limited understanding goes.

Again, I don't plan on shooting large game with the ELDM but here's some reference material:

 

[JakeC]

first you have to decide what is the max length you can get in your mag or going single shot.
while .090 seems short you really need to keep that for a short action with a 2.9 mag but if using long action or singleshot you can throat it with a .225-.250 freebore. mine has a .228 and while good for bergers and some bullets in the 160-168 class bullets you really need .245-.250 for the elds. all the different bullets will end up 3.0 -3.150 oal. with the .225 and longer freebores.

best bet is a dummy round at max mag length so your distance off lands will leave room in mag.

Yes it's definitely going in the magazine. my problem is that I've realized I do not know what I'm looking at since based on what I know a .090 freebore leaves me almost 3/16" shy of using the whole mag with a very long match bullet. That just tells me I'm missing something but I'm not sure where. I've read elsewhere that guys are getting 180 bergers to 2.950 in a fresh saami chamber and the difference is nose length isn't enough to account for that much difference based on what I can find.

 

[can1010]

you need to remember the freebore is on a chamber drawing not the case specs. if you have a .090 fb. you need to add the neck chamfer which is .016-.017 then the chamber neck is normally .010 longer so add that and the saami drawing is a 1degree angle so you can add another .015 to that. so add .090+ .016+.010+.015=.131 total now add case length and bullet noes to get a rough oal. that will put you at 2.990 touching now subtract distance off lands and different nose lengths. these # are berger 180 hybrids. you are fine with the reamer you have.
hope that helps

 

[JakeC]

you need to remember the freebore is on a chamber drawing not the case specs. if you have a .090 fb. you need to add the neck chamfer which is .016-.017 then the chamber neck is normally .010 longer so add that and the saami drawing is a 1degree angle so you can add another .015 to that. so add .090+ .016+.010+.015=.131 total now add case length and bullet noes to get a rough oal. that will put you at 2.990 touching now subtract distance off lands and different nose lengths. these # are berger 180 hybrids. you are fine with the reamer you have.
hope that helps

Excellent, that is EXACTLY what I could not determine from the SAAMI chamber drawing. Literally couldn't read what I knew was in there. Thanks friend.

 

[can1010]

glad to help, not to easy in the beginning to determine from a drawing that is why everybody tells you to make a dummy round

 

[Rhovee]

Yes my measurements were to long for 3". They do work great in my 3.2" action. .090 is pretty standard for 2.950 coal

 

[JakeC]

Thanks gents. This was very helpful.

 

[Tyler75]

Can I tag in for a question!
I'm building a 7 saum also. It's on a Tikka t3 and I'm having struggles with figuring the free bore. I can't make a dummy round because I can't find bullets. I want to shoot 155 gr Fed terminal ascent, 162 hornady eld-x, or maybe 155 gr hammer. The barrel will take 4 months so I want to get it ordered. Should I go .090" fb? Or stick with saami?

 

[can1010]

Can I tag in for a question!
I'm building a 7 saum also. It's on a Tikka t3 and I'm having struggles with figuring the free bore. I can't make a dummy round because I can't find bullets. I want to shoot 155 gr Fed terminal ascent, 162 hornady eld-x, or maybe 155 gr hammer. The barrel will take 4 months so I want to get it ordered. Should I go .090" fb? Or stick with saami?

the Tikka can have the bolt stop replaced to give a longer oal. I would run a longer freebore and take advantage of the extra case capacity. the bullets you listed are not jump sensitive so it wont hurt you but still will give you the option to run the longer high bc. bullets at the 3.1 oal. in the future if you choose