When a rebarrel is specified, most of the conversation goes to steel, twist and contour. Then someone asks what chamber, and the answer decides something the other specifications cannot: what the rifle will physically accept, how it handles pressure, and whether it extracts cleanly. Chamber specification is also where the most persistent myth in precision shooting lives, the idea that a tighter chamber is automatically a more accurate chamber. It is not automatic. It is a trade, and knowing which side of it your shooting sits on is worth more than any brand of reamer.
A chamber reamer cuts four things that matter: body diameter, throat length, leade angle, and neck diameter. Body diameter controls how much the case can expand and how easily it extracts. Throat length and leade angle control how far the bullet travels before it engages the rifling. Neck diameter controls how much grip the neck has on the bullet before the case releases it. Factory SAAMI dimensions are deliberately generous, because the chamber has to accept any compliant ammunition from any maker, including ammunition made on a Monday in a factory that was not paying attention.
A SAAMI-spec chamber is the safety margin of the industry. The dimensions are cut to accept worst-case ammunition, extraction is easy even with dirty or slightly over-pressure brass, and feeding is reliable. Accuracy is not sacrificed in the way forum lore suggests: plenty of SAAMI-chambered factory rifles shoot half-minute with ammunition they like. What a generous chamber costs you is consistency at the top end. More clearance around the case means the case can sit marginally differently shot to shot, and a long throat with a steep leade means the bullet has further to travel before it stabilises against the rifling.
A match chamber tightens the body, shortens the throat and reduces neck clearance. The point is not magic accuracy, it is reduced variation: the case seats more consistently against the bolt face, the bullet starts closer to the rifling, and neck tension does the same thing every shot. The cost is that the chamber expects you to hold up your end. Neck-turned, weight-sorted brass, consistent powder charges, bullets seated to a measured datum: that is what a tight chamber is designed around. Feed it mixed factory ammunition with uneven case walls and neck runout and it will neither shoot better nor extract as forgivingly.
The classic mistake is ordering a minimum-spec neck for a factory-ammunition rifle. Factory neck walls vary by several thousandths between makers and lots, and a neck sized for turned brass can grip a thicker factory case so hard that pressure rises past the intended curve. You will see it on the brass before you see it on paper: cratered primers, sticky lift, case head expansion. If the plan is to shoot factory ammunition, the chamber needs a factory neck, full stop. If the plan is to neck-turn brass, measure the brass you will actually use before the reamer is ordered, not after.
A chamber is not a fixed thing. The throat erodes with round count, moving the rifling engagement further from the case and changing pressure and velocity, and this happens faster in overbore calibres. A chamber that shot brilliantly at 300 rounds is a different chamber at 1,500. The practical consequence for a rebarrel is that specifying a chamber around its day-one dimensions ignores where it will spend most of its life. The signs of a throat that has moved too far are covered in the piece on throat erosion and accuracy loss.
The cartridge itself narrows the choice more than most shooters expect. Overbore calibres with long powdered columns, the 6.5-284 family, the big 7mm magnums, sharpen every chamber error because they run hot and erode fast, and a tight chamber in one of these is a chamber that will need rescoring sooner. Mild, efficient cartridges tolerate generous or tight dimensions with fewer consequences. Legacy military calibres come with their own consideration: surplus ammunition varies enormously in case dimension and pressure, and a match chamber in a rifle intended for surplus ammunition is a combination that will show you pressure signs within a box. The reamer should be chosen for the cartridge's behaviour across its whole life, not just its ballistics on paper.
Two questions settle it. First: will this rifle ever need to fire ammunition you did not load? If yes, and for most working rifles the honest answer is yes, then SAAMI or a mildly tightened variant is correct. A hunting rifle that might one day fire a borrowed or shop-bought round is not the place for a match neck. Second: how much brass preparation are you genuinely going to do, every time, for the life of the barrel? Be honest. If the answer is sorting and turning once in a while, the forgiving chamber will outperform the tight one in your hands, because consistency of process beats tightness of steel.
The customers who end up unhappy are rarely the ones who chose the conservative chamber. They are the ones who read about match chambers and ordered one for a rifle they feed from a shop shelf. Chamber spec should match your ammunition discipline, not your ambition.
If you are planning a rebarrel, include the ammunition question in the enquiry and the chamber gets specified around how you will actually feed the rifle. The rest of the job, from threading to headspacing, is documented on the process page, and the factors worth weighing before steel is even ordered are covered in choosing barrel steel.