Yes, and NEF is not only a quality brand (as a part of Marlin), but the design is a minor modification of the H&R Topper which has been around for decades. .223 is also a very popular chambering for it, as are a number of much higher pressure rounds. If it can not survive 5.56mm milsurp ammunition pressures, then how can it handle.30-06? I ask that out of pure curiosity, as a friend has a .30-06 barrel for his, and has owned it for several years.

I refer you to my post. This is not an issue inherent in the design of the firearm. This was most likely an ammunition failure, possibly caused by over loaded or low quality cases. The odds of a quality control failure in the production of the firearm and barrel are remote, but not outside of the realm of consideration. Another potential is a problem that I've seen with VERY old Toppers, where the action seems to be locked, but isn't, which causes the action to open during firing; while it is extremely unlikely in this case, due to the age of the firearm, it is possible that there was a lock failure due to a malformed part or maintence on the part of the prior owner.

The reason why I say this NOT a 5.56mm vs .223 Rem issue is becuase of the sheer number of of these guns which have been fitted with the .223 Remington barrel, and virtual gaurentee that they will be shot with milsurp ammo (inexpensive gun, inexpensive practice ammo- notice the similiarities?), means that we would have heard about this long before now if the NEF actions and barrels weren't able to handle the milspec ammo.

I said "just about" and "should", becuase there are some very light weight and/or lesser quality frames and mechanisms that have no purpose even trying to handle the .223, such as the various derringers made for it, and becuase some of the vary early .223s and .308s were on the market before the details of the military ammunition was widely available to the public. It also may apply to the various .223 break action conversions for the 1911 that have been produced. BUT I'll go so far as to say ANY rifle chambered for the .223 Remington cartridge manufactured in the past 25 years, and having met approval of both federal and industry regulatory agencies for commercial sale, can and will fire the 5.56x45mm NATO specification round (even if you have to hand feed it), baring catastrophic and freak failure of the cartridge and/or the gun due to issues arising from insufficent quality control. Any time a manual says that you should not fire military specification ammunition, it's in there due to the insistance of lawyers.

While there may be some failures to ignite (due to harder than standard primers), to feed (silly, over long bullets, but that would apply only to a VERY old .223, and never to a break action) or to properly eject (due to very slight case swelling, which would be no worse than experinced on the H&K roller locks), when using military ammunition, the firearm should be able to safely handle that. Other than those points, the whole "military vs similiar civilian" ammunition is anachronism for modern firearms, and mostly stems from when the military had not yet adopted smokeless powder as a propellent, but the sporting shooters had. It was furthered when sporting rifles were produced for the .30-03 cartridge, which not only had a round nosed bullet but also a significantly lower chamber pressure than the .30-06 that used the same case. (A similiar issue led to the .development of the.32 Special, vs .30-30 which uses the very same case.) When fired out of a well made bolt action, such as the Spingfield, they were fine, but when fired out of a rebarreled Krag or one of the many cheap (not inexpensive, but truely cheap) breakactions of the era, the .30-06 would have an extremely negative effect on the action. But for most modern production weapons, this issue is mostly an urban legend which the lawyers keep alive.

Martin's accident is a fluke, but the point you are looking at hasn't been an issue for decades in all but a tiny number of cases. Any action which the manufacturer says will handle .30-06 is not one of those cases.