Exploring mono-bloc, demi-bloc and chopper lump barrels
It is, at times, easy to take for granted the complexity of manufacturing involved in creating a fine sporting rifle or shotgun. As hunters or shooters, finding a gun that fits, breaks clays or kills birds is foremost in our mind. As we dive into and explore the construction of modern guns, we quickly begin to see a great diversity of techniques developed over the last 200 years or so that illustrate just how much thought has been put into the construction of the shotguns and rifles we love.
With the exception of double rifles, which fall into the larger category of ‘double guns’ a vast majority of modern rifle barrels are fitted to their actions via barrel threads. The threaded barrel is similar to a screw that threads into the same pitch threads in the action. A gunsmith will painstakingly cut these threads until proper headspace, or fit between action, bolt and barrel is achieved.
Double guns are in general manufactured differently, although there are always exceptions. Break action doubles like your Beretta 686, Parker or Holland and Holland double rifle all have barrels that have been jointed together without threading a barrel.
In terms of geometry, if we think about any gun with two barrels, it may be a first intuition to think that the bores are parallel to one another. This is in fact not the case. For both double rifles and shotguns, the barrels are laid such that the trajectory of a bullet or shot from the bores crosses that of the opposing barrel downrange at some specified distance. This convergence allows a single targeting sight plane to function for both barrels. In terms of double rifles, this is exceptionally important and ‘regulation’ of these guns is perhaps one of the most mythological and mystifying procedures I have heard folks speak about in the gun world. There are very few folks in the states that will even take on the task of regulating a double rifle, the process of ensuring a proper cross at a specific distance with particular ammunition (side by side or over/under).
Terminology of Jointing Double Guns
Needless to say, jointing double gun barrels happens in two ways, but terminology first. The breech end of the barrels is commonly called the breech bloc or lump, though the ‘lump’ or ‘lumps’ may refer to individual surfaces that lock an action shut. What I will refer to as the lump or breech bloc contains the lockup surfaces, ejector/extractor channels, the breech face and all associated elements. The lump can be created from a single solid piece of steel without barrels connected upon which barrels are added after machining the lump by sleeving and braising. Or the lump can be formed during the jointing process. That is to say that each barrel contains a block of metal on the breech end of the barrel half the size of the lump. Those blocks of steel mate together when two barrels are joined and thus the lump is formed. In this case, the lump is machined after the barrels are joined together. In some cases, the barrels are separate pieces and a third piece is joined to create the lump illustrating the diversity of manufacturing possibilities.
Mono-bloc barrels are formed by machining the chamber portion or lump from a single solid piece of steel by take advantage of modern precision engineering and manufacturing techniques. The biggest advantage is allowing the complex (and very co-dependent) angles to be machined into the action with a high degree of accuracy. The final fitting required on a mono-bloc gun is typically less than others. These guns can usually be produced with less final hand fitting which can lower manufacturing cost (but not necessarily quality).
Demi-bloc and Chopper Lump
There are two styles of jointing barrels that utilize full-length single-piece barrel + lump construction. Demi-bloc and chopper lump barrels both involve joining two halves of the barrels, top and bottom for an O/U and each side for the side x side, to make the barrel set. Demi-bloc barrels utilize a male/female dovetail to mate the two barrel halves. Chopper lump barrels simply mate two flat surfaces in the action end of the barrels to form the lump. Shoe lump or through lump barrels are a third option where two full-length barrels are joined with a third machined piece that contains the ‘lump’ or ‘lumps’. Demi-bloc and chopper lump barrels are often times confused with one another and a host of marketing folks have helped to confuse the topic more by calling one the other and so forth.
How They Compare
When considering the pros and cons of each of the above, it is commonly accepted that Demi-bloc barrels are the strongest being made of only two full-length pieces and joined by a dovetail. Chopper lump barrels were developed heavily in British guns and produce the thinnest and lightest barrels while maintaining strength. They are also the most time consuming and difficult to produce but considered the finest in construction.
Through lump are common in some of the American classic doubles that we all know and love, though they along with shoe lumps generally tend to be wider and heavier in construction. Finally, many modern guns take advantage of CNC machining technology and utilize the faster production process offered by mono-bloc barrel construction, particularly in over/unders.
As with dog breeds, E-collars and upland vests, each has its own benefits, each its own detractors. All of the above-described methods of manufacture have produced successful, strong and well-built firearms though there are some clear winners in terms of strength and time/cost of manufacture. For those wishing to dive into the depths of shotgun technica, hopefully, this illustrates the complexity of a potentially seemingly simple topic like slapping two barrels together.