Why do some rifles consistently shoot sub-MOA groups with just about any ammo and others struggle to get a 2-MOA pattern with meticulously worked-up handloads? While there are numerous issues that affect and degrade accuracy, most of the time, it boils down to the quality and condition of the barrel. I am somewhat reluctant to make this statement without a whole bunch of qualifiers, but, the quality and condition of the barrel is the single most important component affecting accuracy.
What Makes a Good Barrel Good?
A quality barrel starts with highly uniform superior raw materials (stainless or chromoly steels typically) and then machined to extreme tolerances under a very rigorous quality control process. With todays precision machines and highly refined manufacturing techniques as well as the quality of materials, most of the volume barrel manufactures produce very good barrels. This is evidenced by the very respectable, out of the box, accuracy of the current crop of inexpensive rifles like the Ruger American. This was just not the case even 15 or 20 years back.
There are a few distinct manufacturing processes used to produce barrel features. Hammer forging, direct cut or button rifling techniques are the major ones and are hotly debated amongst shooters and manufactures as to which is the superior method. I have built a lot of rifles using barrels produced with all of these techniques and find no clear winner as far as accuracy and general performance. Personally, I prefer cut rifling which is then hand lapped to final dimension, but I am not militant about this. Perhaps half the rifles we build are with lower costs button rifled barrels, all with exceptional results.
Over the past few years we have begun building most of our higher end rifles using carbon fiber wrapped barrels sourced from Proof Research and Paradigm. If you are willing to pay the almost $1,000 for a blank, in my experience, these are best barrels on the market...hands down! These barrels are not only very accurately produced and deliver extremely good performance, you get the rigidity of a heavy bull barrel with the weight of a sporter couture. This allows for more successive shots downrange with little to no point of impact shifts which are very common in lighter couture traditional barrels.
What Else Can Effect or Contribute to Accuracy Problems?
This is a huge topic and can easily get out of hand and lengthy so I will just focus on some of the most common mechanical system issues and not get into any of the ammo/cartridge and bullet specific nuances and debates. The best way I can think of to approach this topic is to describe our approach to evaluating a bolt action rifle that comes into our shop with accuracy problems. Depending on what the cartridge the rifle is chambered in, if a hunting rifle can't maintain at least a 2-MOA 3-shot group at 100 yard, with quality factory ammo, this falls outside of acceptable accuracy and should be corrected.
First, assuming the gun is in good working order and has been reasonably well maintained with no obvious signs of damage (crown defects, etc.), we inspect the scope and mounting system. This typically involves an inspection of the mounting points of the rings and bases to ensure everything is tight and torqued to spec and there are no signs that the scope is moving in the rings under recoil. From there we move on to the scope itself and perform a few tests to ensure the internals are intact and performing as they should.
Once we eliminate the sight system, we move on to disassembling the rifle to look for signs that action may be moving in the stock or that the barrel is contacting the fore end inconsistently (we generally like to free float barrels but if not, contact must be uniform).
Next we check headspace and scope the bore looking for throat erosion, pitting, heavy fowling and muzzle wear. After this we inspect the bolt, bolt face and locking lugs for proper alignment and signs of proper mating. You would be surprised at the number of new rifles that we evaluate where only one of the bolt lugs are making contact on the internal locking lugs.
If all of these systems check out and fall within acceptable limits, then 99% of the time it boils down to a poorly manufactured or machined barrel. I have had cases where we get to this point and when we pull off the barrel we discover that the tenon threads were undersized or malformed and epoxy was used to tighten up the joint...I have seen this many times and expect this may be the culprit in many accuracy problems, but either way you are looking at a re-barrel job to fix it.
We generally recommend glass or pillar bedding and barrel floating when we build or re-work a rifle, but these improvements are not typically the sole solutions to accuracy problems.