This may be a stupid question, but I'm having trouble zeroing my M99 with a Larue mount holding a Leupold scope.  I run out of clicks just as I barely get it lined up.  I understand the Larue mount has 30 MOA of cant, and the M99 rail has an additional 27 MOA.  I don't think I'll be shooting much past 200 yards any time soon, so 57 MOA is excessive for me.  In my head if I run my 30 MOA Larue mount backward on my 27 MOA rail, by my math I should have 3 MOA of negative cant, which I should be able to overcome with the scope.  Not ideal, I know, but is this a solution or am I not considering something?

The quick answer is "sure you can".

But if the rail is removeable on the M99, and flat on the bottom, I'd take some 80 grit wet/dry sandpaper on a flat surface
and lap some of the slope out of the bottom of it.  One minute of an angle (1moa) is a tiny fraction of an inch on a 6 or 8 inch long surface.

On a 6 inch surface 1moa = .0017  ...one point seven thousandths of an inch

On an 8 inch surface 1moa = .0023 ...two point three thousandths.

Calculate how much you need to get a zero 5moa? 10?...
and multiply it by the correction factor.

Measure the thickness of your rail and subtract the result of your multiplication above.

Put the 80 grit on a flat surface with a little oil on it and press more on the REAR portion of the rail when lapping
to bring the scope elevation back up.

If you need more than ten or so thousandths, especially if the rail is steel, have a machinist skim the bulk of the material off for you
...unless you're really determined, have a lot of time, or like sore fingers.  :D
(Bring him a set of rings with straight sides so he can clamp to those in his vise)

By doing all that you'll get the most out of your scope if/when you get a chance to really stretch out its (and your) abilities.

I am going to leave it as it is for it's first range trip tomorrow.  Today's result was just based on a ~25yd boresight, hence why I'm not going to get too crazy.  I'll try to boresight it at 100 tomorrow, and shoot it.  If I can get it on I'll leave it as is.  

The other option would be to contact Barrett and see if I can buy the 0 MOA rail that they made/make for the M99.  I know the older rifles came from the factory equipped with this rail as OE, so perhaps I can get one of those.  I really like the Larue mount, so I'm inclined to keep it if at all possible.

There isn't a 3rd party rail cheaper than what barrett will charge?

For $20 any competent machinist would be happy to custom-slope a small flat surface for you +/-.001

Would likely be the easiest/quickest $20 he made all day.
That would get him well within 2moa of his targeted slope,
Which is easy to lap-in from there.

But he doesn't want that.


I suspect this will go on for three pages...
That is what it took to get a primer pocket reamed to depth and chamfered.
(Which is the same level of difficulty as this)

Thanks for the vote of confidence.  I'll take a look into having my existing rail machined flatter.  Is removing the rail on the M99 as simple as removing a few screws, or is there something more complicated I should look out for?

I'd reverse the mounts before spending money.

Well, it looks like things will work out.  I shot the rifle today and was on paper, albeit without much to spare.  I didn't have a whole lot of time today, and half of the time I did have was spent cleaning the barrel for break in.  Next time out I'm going to have to work on the zero a bit more.  This is the first 50 I have shot in probably 10 years.  I completely forgot what an experience that was!  I'm looking forward to a long future with this rifle!

Edited. Keep it clean, no personal attacks on the forums.

Still have that big shit eating grin? The 1st shot is big woody time. Shot mine last weekend to test some reloads. Just love that big boom. Hope to zero at 500 yards next month.

Come here, in a technical forum, ask questions, don't listen/take the advice, get schooled!

I don't post up here unless I KNOW the answer.

It's experience. It's worth something. Dont want it...?  FINE.

THAT attitude is why you can count on the fingers of one hand the number of old salts on this forum.

I've probably spent $100k+ and over a decade coming up with my experience I try to pass on
I'm not here to waste your time.

If someone asks "what should I do?", and you tell them, and then they don't, it gets just a little frustrating.



Good luck with that.

I apologize if I came off a bit harsh before.  I'm trying to coach you in a big-league game.  Maybe I take it a little too serious.
But to me it is serious stuff.

It is important to model youself after successful shooters...
Every one uses a stainless bristle brush on a '50.

I'm a big fan of the bore cleaning foam solvents -but they aren't enough in-and-of themselves.

It takes the scrubbing only a (fresh) stainless brush can provide to get that big hole clean.

For break in I, after cleaning thoroughly, I'm a big fan of "minor lapping" with a tight jag
with a patch and some Mothers Mag and Aluminum polish (or Wenol, or your favorite)
By smoothing out the bore (which is what you're trying to do with a good break in) you'll get
less copper fouling and a more consistent shooter.

When you don't get much "blue" out of the foaming solvent you know you've gotten what you wanted to do.

A good eye will tell you the bore is smooth.
The light blue/not greenish solvent will be proof of that.

Some bbls take 20 rounds to be "free and clear", most take a hundred or two.

The important thing is you're trying to smooth the internal bore without having copper buildup.

A smooth bore without copper buildup will keep copper buildup from happening in the future.
Which keeps it all the same.
Which keeps it consistent.

Which gives you a stable/repeatable platform to work on the tough part.  You.  :)

No worries, I have a thick skin and appreciate the help.  I didn't see much copper fouling with my cleaning yesterday, mostly just black soot.  I was using Hoppes Elite copper terminator solvent.  I'd soak a patch in it and run it down the bore, let it sit for a few minutes, scrub with a bronze brush (looks like I may need to invest in SS?) and them another patch with solvent, followed by CLP patches until clean.

I've got a carbon cleaning rod with a brass attachment tip-- any issues with this solvent?  Also, which brush do you recommend?  I'm having a hard time finding a 50 caliber SS brush, almost everything out there is bronze.

Brownells sells 50BMG SS Cleaning brushes.

I recommend them, but have had some pull out of their ferrules when back-stroking.
NEVER change direction while in the bore of the rifle.

These brushes  work well as broken shell extractors if back stroked in neck of broken case.

I use 28% aqueous ammonia to remove any brass or copper from bore alternating with a good carbon remover for burnt powder residue.
A good quality barrel properly broken-in cleans easily.

Clean outside in fresh air when using ammonia = or it could kill you.

Those are the exact brushes I use. Come in a multi (10? 12?) Pack.  They are excellent.

I never thought of using one as a shell extractor.  GREAT idea!

...I've never had one fail back-stroking, but have using them in a power drill for other stuff.
(Once you have a bag full of them they come in handy for all sorts of things...)