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Posted: 4/3/2008 7:06:09 PM EDT
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What is the recommended headspace size for a NM chamber? I have a set of headspace guages from Brownells that I will be using. Thanks. |
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The head space stays the same (sideways and to the last chamber cut). What is different is the throat is reamed for the bullet type to be used. So if you are going to run 168 BTHP's (longer ogive), the throat would be cut shorter to prevent a lot of jump to lands when fired (read .003 jump would be good for a standard OAL round since the round would fit/cycle out of a mag, and not have a lot of jump to lands). Note: this is done with two reamers. The chamber is cut, then the throat is cut for the given round type. |
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Standard 308/ 7.62 ball reamer is going to throat deeper than you want if you are shooting 168's. With these chambers, you may find that when you set the bullet .003 off the point of bullet embed at loading, the round will be too long run out of the mag. BT design bullets don't like to jump (read groups start to go down hill after about .008 jump or more), and if that is the case, then flat base bullet types will be better for a longer jump when mag loaded/feed. Again the NM chamber it's self if just a tight 308 chamber that will just accept a go-gauge (nothing special here), and the real math/NM part comes in is throating cut so a standard length 168 loaded round will mag feed, and the bullet ogive .003 off the lands with the bolt locked up. |
As I understand this post, you are saying you want very little leade in the throat area when chambering a match grade barrel for best accuracy and if using boat tail bullets? I appreciate this info a great deal. I normally don't worry about bullet seating or reloading for my M1As, but as expensive as top grade match ammo is now, I'm starting to reload for my NM M1A. |
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You do not want the bullet to embed in the rifling at loading. Doing this will cause a huge pressure spike as the round is fired. Some barrels are forgiving and will allow a jump of up to .010 or larger before accuracy starts to degrade (blow by getting past the BT base and reaching the barrel/rifling before the bullet does). Others are not, and you may even run into embedding out of true, or bad deformity's when doing so with too much jump from these. As for how you throat a barrel for factory ammo, base the throat distance from the last chamber cut for the steepest Ogive bullet design that you are going to use. When using a longer ogive bullet design at the same loaded OAL, there will be a greater jump to the lands on these other selected ammo's. As for a factory 7.62 or 308 standard reamers that throats at the same time, don't be surprised if you find that the 168 bullet has a .080 jump or more when loaded to standard lengths (to fit/cycle from the mag). |
I close my bolts on a bullet seated way too long (unprimed case), then adjust my seating die down a bit to prevent the bullet from hitting the lands to prevent that high pressure. You can't obviously do that on a M1A. Then you have the maximum seating length of 2.8". Do you think it is just best to seat all bullets to that length and not worry about the bullet jump? Thanks! |
Why not, the head space is going to come off the chamber shoulders if chambered reamed correctly, and either a Stoney gauge or smoking the bullet to check for engagement/needed OAL for correct jump. This is pretty much the same manner that you would go after a bolt action rifle, since if you checked for bolt handle resistance only, then you still may be slightly embedding on lock up with a fire formed case when tayloring a reloaded round. Bottom line here is to remember that on an auto loader, you will be working/loading from a mag, and having any OAL ammo/throat longer spec'd (bullet .003 off) than that will not fit through the mag will require the round be sled'd when feed through the rifle. If you looking for a standard chamber that will allow 147 ball to loaded/feed as such, then don't expect 168's to even get close since the ogive is much longer on that bullet (read will have to sled for such to get the most out of the round. Think of it this way, you pick the barrel twist. production type (broach/ button/ hammer forged) and even landing (amount of lands/grooves) for a specif bullet type/range requirement when selecting a barrel, so why not produce the chamber/throating for the same ammo/bullet type as well when completing the barrel portion of the NM rig build. |
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