somebody has to say it, so i will.
the location of the shop will determine your external and internal security concerns. a shop in a strip mall in the suburbs can be very different from one in a shady downtown area, and still be secure. clientel demographics will determine how displays need to be arranged in the interest of loss prevention. this, in turn, will affect the general structure of the shop.
in a general sense, you need a masonry construction, concrete filled steel bollards in front of any plate glass or entryways, security for any such glass, grids made from 3/4" steel round bars in any ventilation opening. combine these with a good alarm system and you have no need of a secure vault. you also need to meet fire code.
you need to understand how big the store will be and how it will be staffed in order to know how to set it up. you need to know how much inventory will be held in order to reserve display space. you need to know if it will be a gun store with a few fishing poles, or a "sporting goods store" full of chinese made clothing with a few guns in the back.
lets talk about a simple 4 person staffed strip mall location layout:
a gunstore with related supplies and a range should have at least 4-5 persons on staff. ideally, it should have retail space 3 times as deep as it is wide. it should have a single securable street front entry. it should have strong lighting, and good air circulation, with no dark corners. you should be able to see from one end to the other, from just about any position in the showroom floor.
if in a strip mall the sidewalls need to be made resistant to break in. if the existing walls are just steel sheet metal stud and wall board, they must be hardened with masonry or sheet steel (16 -12 gauge, with stitch welded joints) or at least 3/16" X 6" square grid masonry reinforcing wire (welded joints), and overlayed with wallboard. anything less than steel reinforced and poured walls can be cut thru in 15 minutes with the right tools, but anything less than what i recommend is irresponsible and won't impress ATF or your insurer.
in the front half of the store there should be a cashier station close to the door, then fluff accessories like clothing and gun cases, then bulk stuff, like shotshells and reloading supplies. the back half of the store should have low racks in the center of the floorspace for high dollar stuff in small packages, like centerfire ammunition, reloading tools, leather goods, etc.
the back half of the store should have a U shaped counter for handgun and optics displays, with a 3 foot walkway behind for clerks, and wall racks behind this walkway for long guns, with shelves underneath for storage. 2 clerks stationed left and right can keep an eye on the entire rear of the store, including all of the high dollar inventory. a possible 3rd gun clerk can run a cashier station between the front door and the gun sales floor, and reduce loss by theft, if the center racks are oriented correctly.
there should be a single door leading out the back wall of the retail space into a viewing area from which the range can be seen thru bulletproof lexan. the range should be set up so that shooting is done towards a hardened back wall. sidewalls need to be adequate, as does the roof and floor. ventilation of the range will be controlled by EPA regulation.
a registration counter can be built to one side of the viewing area. targets and range ammo can be sold there. entry to the firing area can be controlled by the counter clerk thru an electric lock, as the clerk can see all lanes and the free space of 6' depth behind them. exit from this area must be unrestricted, so the lock is a one way thing. the clerk working the range can be occupied on the showroom floor when the range is empty.
there should also be an emergency exit accessible from this viewing area. it can be a hallway running to the back of the building parallel to the shooting range. egress thru this path cannot be restricted per fire code. locks and alarms, including "opened from the inside" and break in alarms with different tones, special night lock, of course.
raising the gun sales area a few feet and having some 3-4 steps in the center up to it gives you an excuse to seperate it from the front half by reinforced glass or steel "jail bars" with a lockable door that can reduce the size requirement for your secure area, and thereby, construction cost. a securable door between the gun sales area and the range viewing area could be the back wall of your secure area. a raised floor gun sales area can also give your gun sales clerks an overview of the soft goods floor.
a lounge area for general BS'ing can be created at the foot of the steps to each side with a few couches and coffee tables and a TV playing hunting/shooting shows. a fishing goods section can be added between soft goods and the lounge, but requires another staffer.
an office space? a gunsmithing room with enuf equipment to mount rifle scopes and install trigger groups, and to clean and make minor repairs (drill press and workspace with counters and cabinets, vise)? make the passage back to the range viewing room a hallway, and build these in left and right (150 square feet each, minimum). make them accessible from the hallway, or better yet, from behind the sales counters. that's 2 more hardened doors in your secure perimeter...
you really need to talk to your customer, make up sketches for a strip mall conversion, new construction, or rehab of some freestanding building, then talk with him some more. security systems, cameras, phone lines, internet lines, gotta have it all. then you have to find a location with appropriate zoning. no matter how cheap a building in a less desireable area looks to be, it's a dead end. people with money to spend just don't wanna go there, and loss prevention and general risk will be a killer.
this ain't gonna be cheap, and he'll realise it when he sees the scope of the work involved in just converting a strip mall shop, which is much cheaper than building new construction. adding the range to the shop will cost a lot more in terms of liability insurance, zoning regulation, environmental considerations, construction, etc. and may limit your choice of suitable property. on the flip side, it will bring in customers with money to spend.
i wouldn't waste time on anything more than preliminary sjetches and cost estimates till he gets the money lined up and finds a place he can operate at for a minimum of 15 years. it'll take him that long to get out of the hole, if things go really well.